diff --git a/LICENSE b/LICENSE
index f288702..dbaf0c1 100644
--- a/LICENSE
+++ b/LICENSE
@@ -1,674 +1,201 @@
-                    GNU GENERAL PUBLIC LICENSE
-                       Version 3, 29 June 2007
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-  If you develop a new program, and you want it to be of the greatest
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-  To do so, attach the following notices to the program.  It is safest
-to attach them to the start of each source file to most effectively
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-    This program is free software: you can redistribute it and/or modify
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+                        http://www.apache.org/licenses/
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+   5. Submission of Contributions. Unless You explicitly state otherwise,
+      any Contribution intentionally submitted for inclusion in the Work
+      by You to the Licensor shall be under the terms and conditions of
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+   6. Trademarks. This License does not grant permission to use the trade
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+      Contributor provides its Contributions) on an "AS IS" BASIS,
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+      whether in tort (including negligence), contract, or otherwise,
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+      negligent acts) or agreed to in writing, shall any Contributor be
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+   9. Accepting Warranty or Additional Liability. While redistributing
+      the Work or Derivative Works thereof, You may choose to offer,
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+      of your accepting any such warranty or additional liability.
+
+   END OF TERMS AND CONDITIONS
+
+   APPENDIX: How to apply the Apache License to your work.
+
+      To apply the Apache License to your work, attach the following
+      boilerplate notice, with the fields enclosed by brackets "[]"
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+      file or class name and description of purpose be included on the
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+
+   Copyright 2026 Mikkeli Matlock
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
diff --git a/analysis_results_manager.py b/analysis_results_manager.py
index 76084e1..10194c7 100644
--- a/analysis_results_manager.py
+++ b/analysis_results_manager.py
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
 """
 Analysis Results Manager - Bridge between audio processing and GUI.
 Manages analysis queue and coordinates between components.
diff --git a/audio_visualization_widget.py b/audio_visualization_widget.py
index 0454003..b17bd7e 100644
--- a/audio_visualization_widget.py
+++ b/audio_visualization_widget.py
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
 """
 Interactive visualization widget built on pyqtgraph.
 
diff --git a/font_manager.py b/font_manager.py
index 633c939..59cd9fc 100644
--- a/font_manager.py
+++ b/font_manager.py
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
 """
 Font management system with CJK fallback support.
 Handles matplotlib and Qt font configuration with licensing-safe approach.
diff --git a/logger_setup.py b/logger_setup.py
index 6671c24..39dd93f 100644
--- a/logger_setup.py
+++ b/logger_setup.py
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
 """
 Logging configuration for the Audio Mastering Toolkit.
 Provides CLI configurable logging with different verbosity levels.
diff --git a/main.py b/main.py
index c8bb5d3..88f69cc 100644
--- a/main.py
+++ b/main.py
@@ -1,420 +1,421 @@
-import sys
-import os
-import logging
-from PyQt5.QtWidgets import (QApplication, QMainWindow, QWidget, QVBoxLayout, 
-                            QHBoxLayout, QSplitter, QLabel, QListWidget, 
-                            QTextEdit, QListWidgetItem, QPushButton, QFileDialog)
-from PyQt5.QtCore import Qt
-
-from audio_visualization_widget import AudioVisualizationWidget, dataset_color
-from analysis_results_manager import AnalysisResultsManager
-from logger_setup import setup_logging, parse_log_args
-from font_manager import initialize_fonts, apply_fixed_font
-from plot_control_widget import PlotControlWidget
-from ref_line_widget import RefLineControlWidget, RefLineDialog
-from metrics import METRICS
-from plotspec import RefLineProps, apply_x_mode
-
-
-class MainWindow(QMainWindow):
-    """Main application window with modular audio analysis interface."""
-    
-    def __init__(self):
-        super().__init__()
-        self.logger = logging.getLogger(__name__)
-        self.analysis_manager = AnalysisResultsManager()
-        # Guards programmatic list mutations from triggering re-render storms.
-        self._suppress_list_signals = False
-        # Reference lines are kept per metric (a -14 LUFS line means nothing on a
-        # spectrogram), so they persist when you switch metrics and come back.
-        self.ref_lines_by_metric: dict[str, list[RefLineProps]] = {}
-        self.initUI()
-        self.connect_signals()
-        self._activate_ref_lines()
-    
-    def initUI(self):
-        """Initialize the user interface."""
-        self.setWindowTitle('Audio Mastering Analysis Toolkit')
-        self.setGeometry(100, 100, 1200, 700)
-        self.setAcceptDrops(True)
-        
-        # Create central widget with splitter
-        central_widget = QWidget()
-        self.setCentralWidget(central_widget)
-        
-        # Main horizontal layout
-        main_layout = QHBoxLayout(central_widget)
-        splitter = QSplitter(Qt.Horizontal)
-        main_layout.addWidget(splitter)
-        
-        # Left panel - File list and metadata
-        left_panel = self.create_left_panel()
-        splitter.addWidget(left_panel)
-        
-        # Right panel - Visualization
-        self.visualization_widget = AudioVisualizationWidget()
-        splitter.addWidget(self.visualization_widget)
-        
-        # Set splitter proportions
-        splitter.setSizes([300, 900])  # Left panel narrower than visualization
-    
-    def create_left_panel(self):
-        """Create the left panel with file list and metadata."""
-        panel = QWidget()
-        layout = QVBoxLayout(panel)
-        
-        # Open File button
-        self.open_file_button = QPushButton("Open Audio File...")
-        self.open_file_button.clicked.connect(self.open_file_dialog)
-        layout.addWidget(self.open_file_button)
-
-        # Plot control cluster (metric selector + scale toggle + refresh)
-        self.plot_control = PlotControlWidget()
-        self.plot_control.metricChanged.connect(self.on_metric_changed)
-        self.plot_control.viewChanged.connect(self.on_view_changed)
-        self.plot_control.plotRefreshRequested.connect(self.on_plot_refresh_requested)
-        layout.addWidget(self.plot_control)
-
-        # Reference-line management cluster (list + add/edit/remove/clear).
-        self.ref_line_control = RefLineControlWidget()
-        self.ref_line_control.addRequested.connect(self.on_add_reference_line)
-        self.ref_line_control.editRequested.connect(self.on_edit_reference_line)
-        self.ref_line_control.removeRequested.connect(self.on_remove_reference_line)
-        self.ref_line_control.clearRequested.connect(self.on_clear_reference_lines)
-        layout.addWidget(self.ref_line_control)
-
-        # File list. Each item carries a checkbox: the checked set is the overlay
-        # set drawn on the graph; the highlighted item drives the metadata panel.
-        self.file_list_label = QLabel("Analyzed Files (tick to overlay):")
-        layout.addWidget(self.file_list_label)
-
-        self.file_list = QListWidget()
-        self.file_list.itemClicked.connect(self.on_file_selected)
-        self.file_list.itemChanged.connect(self.on_file_check_changed)
-        layout.addWidget(self.file_list)
-        
-        # Metadata display
-        self.metadata_label = QLabel("File Information:")
-        layout.addWidget(self.metadata_label)
-        
-        self.metadata_display = QTextEdit()
-        self.metadata_display.setReadOnly(True)
-        self.metadata_display.setMaximumHeight(150)
-        layout.addWidget(self.metadata_display)
-        
-        # Instructions
-        instructions = QLabel(
-            "Drag and drop audio files (.mp3, .wav) onto this window to analyze them."
-        )
-        instructions.setWordWrap(True)
-        instructions.setStyleSheet("color: gray; font-style: italic;")
-        layout.addWidget(instructions)
-        
-        return panel
-    
-    def connect_signals(self):
-        """Connect analysis manager signals to GUI updates."""
-        self.analysis_manager.analysisStarted.connect(self.on_analysis_started)
-        self.analysis_manager.analysisCompleted.connect(self.on_analysis_completed)
-        self.analysis_manager.analysisError.connect(self.on_analysis_error)
-        self.analysis_manager.progressUpdate.connect(self.on_progress_update)
-        self.analysis_manager.metricComputeStarted.connect(self.on_metric_compute_started)
-        self.analysis_manager.metricReady.connect(self.on_metric_ready)
-        self.analysis_manager.metricComputeError.connect(self.on_metric_compute_error)
-        self.analysis_manager.metricTiming.connect(self.on_metric_timing)
-        self.visualization_widget.referenceLineMoved.connect(self.on_reference_line_moved)
-    
-    def closeEvent(self, event):
-        """Stop background analysis/prefetch threads before the window closes."""
-        self.analysis_manager.shutdown()
-        super().closeEvent(event)
-
-    def dragEnterEvent(self, event):
-        """Handle drag enter event for file drops."""
-        if event.mimeData().hasUrls():
-            # Check if any files have audio extensions
-            urls = event.mimeData().urls()
-            for url in urls:
-                file_path = url.toLocalFile()
-                if file_path.lower().endswith(('.mp3', '.wav', '.flac')):
-                    event.accept()
-                    return
-        event.ignore()
-    
-    def dropEvent(self, event):
-        """Handle file drop event."""
-        files = [u.toLocalFile() for u in event.mimeData().urls()]
-        audio_files = [f for f in files if f.lower().endswith(('.mp3', '.wav', '.flac'))]
-        
-        self.logger.info(f"Files dropped: {len(files)} total, {len(audio_files)} audio files")
-        
-        if audio_files:
-            # Analyze the first audio file
-            # TODO: Add support for multiple file queue
-            file_path = audio_files[0]
-            self.logger.info(f"Starting analysis of dropped file: {os.path.basename(file_path)}")
-            self.analysis_manager.analyze_file(file_path, self.plot_control.current_metric_id())
-        else:
-            self.visualization_widget.set_status("No audio files detected in drop")
-            self.logger.warning("No supported audio files found in drop")
-    
-    def open_file_dialog(self):
-        """Open file dialog to select audio files for analysis."""
-        file_path, _ = QFileDialog.getOpenFileName(
-            self,
-            "Select Audio File",
-            "",  # Default directory (empty = current directory)
-            "Audio Files (*.mp3 *.wav *.flac);;All Files (*)"
-        )
-        
-        if file_path:  # User selected a file (didn't cancel)
-            self.logger.info(f"File selected via dialog: {os.path.basename(file_path)}")
-            self.analysis_manager.analyze_file(file_path, self.plot_control.current_metric_id())
-    
-    def on_analysis_started(self, file_path):
-        """Called when analysis starts."""
-        filename = os.path.basename(file_path)
-        self.visualization_widget.set_status(f"Analyzing: {filename}...")
-    
-    def on_analysis_completed(self, file_path, result):
-        """Called when analysis completes successfully."""
-        filename = os.path.basename(file_path)
-
-        # Add to file list (checked, so it joins the overlay set) if not present.
-        item = self._item_for_path(file_path)
-        if item is None:
-            self._suppress_list_signals = True
-            item = QListWidgetItem(filename)
-            item.setData(Qt.UserRole, file_path)  # Store full path
-            item.setFlags(item.flags() | Qt.ItemIsUserCheckable)
-            item.setCheckState(Qt.Checked)
-            self.file_list.addItem(item)
-            self._suppress_list_signals = False
-
-        # Update metadata display and highlight the analyzed file.
-        self.metadata_display.setText(self.analysis_manager.get_metadata_text(file_path))
-        self.file_list.setCurrentItem(item)
-
-        # Redraw the overlay set for the current metric.
-        self._refresh_view()
-    
-    def on_analysis_error(self, file_path, error_message):
-        """Called when analysis fails."""
-        filename = os.path.basename(file_path)
-        self.logger.error(f"Analysis failed for {filename}: {error_message}")
-        self.visualization_widget.set_status(f"Error analyzing {filename}: {error_message}")
-    
-    def on_progress_update(self, message, percentage):
-        """Called when analysis progress updates.
-
-        The messages already carry phase + timing; the percentage was a coarse
-        fake (load jumped 10->done), so it's logged but not shown in the slip.
-        """
-        self.logger.debug(f"Progress: {message} ({percentage}%)")
-        self.visualization_widget.set_status(message)
-    
-    def on_file_selected(self, item):
-        """Called when a file is highlighted (drives the metadata panel only)."""
-        file_path = item.data(Qt.UserRole)
-        self.metadata_display.setText(self.analysis_manager.get_metadata_text(file_path))
-
-    def on_file_check_changed(self, _item):
-        """A checkbox toggled — the overlay set changed; redraw."""
-        if self._suppress_list_signals:
-            return
-        self._refresh_view()
-
-    def on_metric_changed(self, metric_id: str):
-        """Called when the metric selector changes."""
-        self.logger.info(f"Metric changed via GUI: {metric_id}")
-        # Reference lines are kept per metric, so swap in this metric's set rather
-        # than discarding — switch away and back and your lines are still there.
-        self._activate_ref_lines()
-        self._refresh_view()
-
-    def on_add_reference_line(self):
-        """Add a reference line at the current view centre, then edit it."""
-        value = self.visualization_widget.current_view_center_value()
-        props = RefLineProps(value=round(value, 2))
-        self.ref_lines.append(props)
-        self._sync_ref_lines()
-        # Open the editor immediately so colour/tag/value can be set right away.
-        self.on_edit_reference_line(len(self.ref_lines) - 1)
-
-    def on_edit_reference_line(self, index: int):
-        """Open the properties dialog for a reference line."""
-        if not (0 <= index < len(self.ref_lines)):
-            return
-        dialog = RefLineDialog(self, self.ref_lines[index], value_units=self._ref_value_units())
-        if dialog.exec_():
-            self.ref_lines[index] = dialog.result_props()
-            self._sync_ref_lines()
-
-    def on_remove_reference_line(self, index: int):
-        """Delete a reference line."""
-        if 0 <= index < len(self.ref_lines):
-            del self.ref_lines[index]
-            self._sync_ref_lines()
-
-    def on_clear_reference_lines(self):
-        """Remove all custom reference lines for the current metric."""
-        self.ref_lines.clear()
-        self._sync_ref_lines()
-
-    def on_reference_line_moved(self, index: int):
-        """A line was dragged on the plot — its value is already updated; refresh list."""
-        self.ref_line_control.set_lines(self.ref_lines)
-
-    def _activate_ref_lines(self):
-        """Point `self.ref_lines` at the current metric's set and sync the UI."""
-        metric_id = self.plot_control.current_metric_id()
-        self.ref_lines = self.ref_lines_by_metric.setdefault(metric_id, [])
-        self._sync_ref_lines()
-
-    def _ref_value_units(self) -> str:
-        """Units a reference line's value is expressed in for the current metric."""
-        return "Hz" if self.plot_control.current_metric_id() == "spectrogram" else ""
-
-    def _sync_ref_lines(self):
-        """Push the current reference-line set to both the list view and the plot."""
-        self.ref_line_control.set_lines(self.ref_lines)
-        self.visualization_widget.set_reference_lines(self.ref_lines)
-
-    def on_view_changed(self):
-        """Called when a view-scale toggle (lin/log) changes. Recompute-free redraw."""
-        self.logger.info("View scale changed via GUI")
-        self._refresh_view()
-
-    def on_plot_refresh_requested(self):
-        """Called when manual plot refresh is requested."""
-        self.logger.info("Manual plot refresh requested via GUI")
-        self._refresh_view()
-
-    def on_metric_compute_started(self, file_path: str, metric_id: str):
-        """Called when an off-thread metric compute starts."""
-        if file_path not in self._overlay_paths():
-            return  # not in the drawn set; status bar shouldn't lie
-        metric = METRICS.get(metric_id)
-        display = metric.display_name if metric else metric_id
-        self.visualization_widget.set_status(f"Computing {display}...")
-
-    def on_metric_ready(self, file_path: str, metric_id: str):
-        """Called when metric data is available (cached hit or async finish)."""
-        if metric_id != self.plot_control.current_metric_id():
-            return  # user already switched to a different metric
-        if file_path not in self._overlay_paths():
-            return  # no longer part of the overlay set
-        self._refresh_view()
-
-    def on_metric_timing(self, file_path: str, metric_id: str, seconds: float):
-        """An on-demand metric compute finished — report how long it took."""
-        if file_path not in self._overlay_paths():
-            return
-        if metric_id != self.plot_control.current_metric_id():
-            return
-        metric = METRICS.get(metric_id)
-        display = metric.display_name if metric else metric_id
-        self.visualization_widget.set_status(f"{display} computed in {seconds:.1f}s")
-
-    def on_metric_compute_error(self, file_path: str, metric_id: str, error_message: str):
-        self.logger.error(f"Metric compute failed ({metric_id} / {os.path.basename(file_path)}): {error_message}")
-        if file_path in self._overlay_paths():
-            self.visualization_widget.set_status(f"Error computing {metric_id}: {error_message}")
-
-    def _current_file_path(self):
-        item = self.file_list.currentItem()
-        return item.data(Qt.UserRole) if item else None
-
-    def _item_for_path(self, file_path):
-        for i in range(self.file_list.count()):
-            item = self.file_list.item(i)
-            if item.data(Qt.UserRole) == file_path:
-                return item
-        return None
-
-    def _row_index(self, file_path) -> int:
-        for i in range(self.file_list.count()):
-            if self.file_list.item(i).data(Qt.UserRole) == file_path:
-                return i
-        return 0
-
-    def _overlay_paths(self):
-        """File paths whose checkbox is ticked — the set drawn on the graph."""
-        return [
-            self.file_list.item(i).data(Qt.UserRole)
-            for i in range(self.file_list.count())
-            if self.file_list.item(i).checkState() == Qt.Checked
-        ]
-
-    def _refresh_view(self):
-        """Redraw the checked overlay set for the current metric and view-state.
-
-        Renders every dataset whose data is cached; for any that isn't, kicks off
-        an async compute (or a full load if the file was never analysed) and
-        leaves a status note. `on_metric_ready` calls back here when each lands.
-        """
-        paths = self._overlay_paths()
-        metric_id = self.plot_control.current_metric_id()
-        view = self.plot_control.current_view_state()
-        metric = METRICS.get(metric_id)
-        if not paths or metric is None:
-            self.visualization_widget.show_specs([])
-            return
-
-        specs = []
-        pending = 0
-        for path in paths:
-            data = self.analysis_manager.get_metric_data(path, metric_id)
-            if data is None:
-                if self.analysis_manager.is_file_analyzed(path):
-                    self.analysis_manager.request_metric(path, metric_id)
-                else:
-                    self.analysis_manager.analyze_file(path, metric_id)
-                pending += 1
-                continue
-            label = self.analysis_manager.display_label(path)
-            # Colour is keyed to the file's row, not its position in the overlay
-            # subset, so a song keeps its colour as others are ticked/unticked.
-            color = dataset_color(self._row_index(path))
-            spec = apply_x_mode(metric.build_spec(data, view), view.x_mode)
-            specs.append((label, spec, color))
-
-        if specs:
-            self.visualization_widget.show_specs(specs, view)
-        if pending:
-            self.visualization_widget.set_status(
-                f"Computing {metric.display_name} for {pending} file(s)..."
-            )
-
-
-def main():
-    # Parse logging arguments before creating QApplication
-    log_level, log_to_file = parse_log_args()
-
-    # Initialize logging
-    logger = setup_logging(log_level, log_to_file)
-    logger.info("Starting Audio Mastering Analysis Toolkit")
-    logger.info(f"Command line args: log-level={log_level}, log-file={log_to_file}")
-
-    app = QApplication(sys.argv)
-
-    # Initialize font system (loads any fonts/ files, configures fallbacks) then
-    # lock the UI font. M PLUS 1 Code has full Japanese coverage, so this stays
-    # CJK-safe; falls back to the system default if the family isn't present.
-    initialize_fonts()
-    chosen = apply_fixed_font("M PLUS 1 Code", 10)
-    logger.info(f"UI font locked to '{chosen}' at 10pt")
-
-    # Set application style
-    app.setStyle('Fusion')  # Modern cross-platform style
-    logger.debug("Application style set to Fusion")
-
-    window = MainWindow()
-    window.show()
-    logger.info("GUI window displayed")
-
-    sys.exit(app.exec_())
-
-
-if __name__ == '__main__':
+# SPDX-License-Identifier: Apache-2.0
+import sys
+import os
+import logging
+from PyQt5.QtWidgets import (QApplication, QMainWindow, QWidget, QVBoxLayout, 
+                            QHBoxLayout, QSplitter, QLabel, QListWidget, 
+                            QTextEdit, QListWidgetItem, QPushButton, QFileDialog)
+from PyQt5.QtCore import Qt
+
+from audio_visualization_widget import AudioVisualizationWidget, dataset_color
+from analysis_results_manager import AnalysisResultsManager
+from logger_setup import setup_logging, parse_log_args
+from font_manager import initialize_fonts, apply_fixed_font
+from plot_control_widget import PlotControlWidget
+from ref_line_widget import RefLineControlWidget, RefLineDialog
+from metrics import METRICS
+from plotspec import RefLineProps, apply_x_mode
+
+
+class MainWindow(QMainWindow):
+    """Main application window with modular audio analysis interface."""
+    
+    def __init__(self):
+        super().__init__()
+        self.logger = logging.getLogger(__name__)
+        self.analysis_manager = AnalysisResultsManager()
+        # Guards programmatic list mutations from triggering re-render storms.
+        self._suppress_list_signals = False
+        # Reference lines are kept per metric (a -14 LUFS line means nothing on a
+        # spectrogram), so they persist when you switch metrics and come back.
+        self.ref_lines_by_metric: dict[str, list[RefLineProps]] = {}
+        self.initUI()
+        self.connect_signals()
+        self._activate_ref_lines()
+    
+    def initUI(self):
+        """Initialize the user interface."""
+        self.setWindowTitle('Audio Mastering Analysis Toolkit')
+        self.setGeometry(100, 100, 1200, 700)
+        self.setAcceptDrops(True)
+        
+        # Create central widget with splitter
+        central_widget = QWidget()
+        self.setCentralWidget(central_widget)
+        
+        # Main horizontal layout
+        main_layout = QHBoxLayout(central_widget)
+        splitter = QSplitter(Qt.Horizontal)
+        main_layout.addWidget(splitter)
+        
+        # Left panel - File list and metadata
+        left_panel = self.create_left_panel()
+        splitter.addWidget(left_panel)
+        
+        # Right panel - Visualization
+        self.visualization_widget = AudioVisualizationWidget()
+        splitter.addWidget(self.visualization_widget)
+        
+        # Set splitter proportions
+        splitter.setSizes([300, 900])  # Left panel narrower than visualization
+    
+    def create_left_panel(self):
+        """Create the left panel with file list and metadata."""
+        panel = QWidget()
+        layout = QVBoxLayout(panel)
+        
+        # Open File button
+        self.open_file_button = QPushButton("Open Audio File...")
+        self.open_file_button.clicked.connect(self.open_file_dialog)
+        layout.addWidget(self.open_file_button)
+
+        # Plot control cluster (metric selector + scale toggle + refresh)
+        self.plot_control = PlotControlWidget()
+        self.plot_control.metricChanged.connect(self.on_metric_changed)
+        self.plot_control.viewChanged.connect(self.on_view_changed)
+        self.plot_control.plotRefreshRequested.connect(self.on_plot_refresh_requested)
+        layout.addWidget(self.plot_control)
+
+        # Reference-line management cluster (list + add/edit/remove/clear).
+        self.ref_line_control = RefLineControlWidget()
+        self.ref_line_control.addRequested.connect(self.on_add_reference_line)
+        self.ref_line_control.editRequested.connect(self.on_edit_reference_line)
+        self.ref_line_control.removeRequested.connect(self.on_remove_reference_line)
+        self.ref_line_control.clearRequested.connect(self.on_clear_reference_lines)
+        layout.addWidget(self.ref_line_control)
+
+        # File list. Each item carries a checkbox: the checked set is the overlay
+        # set drawn on the graph; the highlighted item drives the metadata panel.
+        self.file_list_label = QLabel("Analyzed Files (tick to overlay):")
+        layout.addWidget(self.file_list_label)
+
+        self.file_list = QListWidget()
+        self.file_list.itemClicked.connect(self.on_file_selected)
+        self.file_list.itemChanged.connect(self.on_file_check_changed)
+        layout.addWidget(self.file_list)
+        
+        # Metadata display
+        self.metadata_label = QLabel("File Information:")
+        layout.addWidget(self.metadata_label)
+        
+        self.metadata_display = QTextEdit()
+        self.metadata_display.setReadOnly(True)
+        self.metadata_display.setMaximumHeight(150)
+        layout.addWidget(self.metadata_display)
+        
+        # Instructions
+        instructions = QLabel(
+            "Drag and drop audio files (.mp3, .wav) onto this window to analyze them."
+        )
+        instructions.setWordWrap(True)
+        instructions.setStyleSheet("color: gray; font-style: italic;")
+        layout.addWidget(instructions)
+        
+        return panel
+    
+    def connect_signals(self):
+        """Connect analysis manager signals to GUI updates."""
+        self.analysis_manager.analysisStarted.connect(self.on_analysis_started)
+        self.analysis_manager.analysisCompleted.connect(self.on_analysis_completed)
+        self.analysis_manager.analysisError.connect(self.on_analysis_error)
+        self.analysis_manager.progressUpdate.connect(self.on_progress_update)
+        self.analysis_manager.metricComputeStarted.connect(self.on_metric_compute_started)
+        self.analysis_manager.metricReady.connect(self.on_metric_ready)
+        self.analysis_manager.metricComputeError.connect(self.on_metric_compute_error)
+        self.analysis_manager.metricTiming.connect(self.on_metric_timing)
+        self.visualization_widget.referenceLineMoved.connect(self.on_reference_line_moved)
+    
+    def closeEvent(self, event):
+        """Stop background analysis/prefetch threads before the window closes."""
+        self.analysis_manager.shutdown()
+        super().closeEvent(event)
+
+    def dragEnterEvent(self, event):
+        """Handle drag enter event for file drops."""
+        if event.mimeData().hasUrls():
+            # Check if any files have audio extensions
+            urls = event.mimeData().urls()
+            for url in urls:
+                file_path = url.toLocalFile()
+                if file_path.lower().endswith(('.mp3', '.wav', '.flac')):
+                    event.accept()
+                    return
+        event.ignore()
+    
+    def dropEvent(self, event):
+        """Handle file drop event."""
+        files = [u.toLocalFile() for u in event.mimeData().urls()]
+        audio_files = [f for f in files if f.lower().endswith(('.mp3', '.wav', '.flac'))]
+        
+        self.logger.info(f"Files dropped: {len(files)} total, {len(audio_files)} audio files")
+        
+        if audio_files:
+            # Analyze the first audio file
+            # TODO: Add support for multiple file queue
+            file_path = audio_files[0]
+            self.logger.info(f"Starting analysis of dropped file: {os.path.basename(file_path)}")
+            self.analysis_manager.analyze_file(file_path, self.plot_control.current_metric_id())
+        else:
+            self.visualization_widget.set_status("No audio files detected in drop")
+            self.logger.warning("No supported audio files found in drop")
+    
+    def open_file_dialog(self):
+        """Open file dialog to select audio files for analysis."""
+        file_path, _ = QFileDialog.getOpenFileName(
+            self,
+            "Select Audio File",
+            "",  # Default directory (empty = current directory)
+            "Audio Files (*.mp3 *.wav *.flac);;All Files (*)"
+        )
+        
+        if file_path:  # User selected a file (didn't cancel)
+            self.logger.info(f"File selected via dialog: {os.path.basename(file_path)}")
+            self.analysis_manager.analyze_file(file_path, self.plot_control.current_metric_id())
+    
+    def on_analysis_started(self, file_path):
+        """Called when analysis starts."""
+        filename = os.path.basename(file_path)
+        self.visualization_widget.set_status(f"Analyzing: {filename}...")
+    
+    def on_analysis_completed(self, file_path, result):
+        """Called when analysis completes successfully."""
+        filename = os.path.basename(file_path)
+
+        # Add to file list (checked, so it joins the overlay set) if not present.
+        item = self._item_for_path(file_path)
+        if item is None:
+            self._suppress_list_signals = True
+            item = QListWidgetItem(filename)
+            item.setData(Qt.UserRole, file_path)  # Store full path
+            item.setFlags(item.flags() | Qt.ItemIsUserCheckable)
+            item.setCheckState(Qt.Checked)
+            self.file_list.addItem(item)
+            self._suppress_list_signals = False
+
+        # Update metadata display and highlight the analyzed file.
+        self.metadata_display.setText(self.analysis_manager.get_metadata_text(file_path))
+        self.file_list.setCurrentItem(item)
+
+        # Redraw the overlay set for the current metric.
+        self._refresh_view()
+    
+    def on_analysis_error(self, file_path, error_message):
+        """Called when analysis fails."""
+        filename = os.path.basename(file_path)
+        self.logger.error(f"Analysis failed for {filename}: {error_message}")
+        self.visualization_widget.set_status(f"Error analyzing {filename}: {error_message}")
+    
+    def on_progress_update(self, message, percentage):
+        """Called when analysis progress updates.
+
+        The messages already carry phase + timing; the percentage was a coarse
+        fake (load jumped 10->done), so it's logged but not shown in the slip.
+        """
+        self.logger.debug(f"Progress: {message} ({percentage}%)")
+        self.visualization_widget.set_status(message)
+    
+    def on_file_selected(self, item):
+        """Called when a file is highlighted (drives the metadata panel only)."""
+        file_path = item.data(Qt.UserRole)
+        self.metadata_display.setText(self.analysis_manager.get_metadata_text(file_path))
+
+    def on_file_check_changed(self, _item):
+        """A checkbox toggled — the overlay set changed; redraw."""
+        if self._suppress_list_signals:
+            return
+        self._refresh_view()
+
+    def on_metric_changed(self, metric_id: str):
+        """Called when the metric selector changes."""
+        self.logger.info(f"Metric changed via GUI: {metric_id}")
+        # Reference lines are kept per metric, so swap in this metric's set rather
+        # than discarding — switch away and back and your lines are still there.
+        self._activate_ref_lines()
+        self._refresh_view()
+
+    def on_add_reference_line(self):
+        """Add a reference line at the current view centre, then edit it."""
+        value = self.visualization_widget.current_view_center_value()
+        props = RefLineProps(value=round(value, 2))
+        self.ref_lines.append(props)
+        self._sync_ref_lines()
+        # Open the editor immediately so colour/tag/value can be set right away.
+        self.on_edit_reference_line(len(self.ref_lines) - 1)
+
+    def on_edit_reference_line(self, index: int):
+        """Open the properties dialog for a reference line."""
+        if not (0 <= index < len(self.ref_lines)):
+            return
+        dialog = RefLineDialog(self, self.ref_lines[index], value_units=self._ref_value_units())
+        if dialog.exec_():
+            self.ref_lines[index] = dialog.result_props()
+            self._sync_ref_lines()
+
+    def on_remove_reference_line(self, index: int):
+        """Delete a reference line."""
+        if 0 <= index < len(self.ref_lines):
+            del self.ref_lines[index]
+            self._sync_ref_lines()
+
+    def on_clear_reference_lines(self):
+        """Remove all custom reference lines for the current metric."""
+        self.ref_lines.clear()
+        self._sync_ref_lines()
+
+    def on_reference_line_moved(self, index: int):
+        """A line was dragged on the plot — its value is already updated; refresh list."""
+        self.ref_line_control.set_lines(self.ref_lines)
+
+    def _activate_ref_lines(self):
+        """Point `self.ref_lines` at the current metric's set and sync the UI."""
+        metric_id = self.plot_control.current_metric_id()
+        self.ref_lines = self.ref_lines_by_metric.setdefault(metric_id, [])
+        self._sync_ref_lines()
+
+    def _ref_value_units(self) -> str:
+        """Units a reference line's value is expressed in for the current metric."""
+        return "Hz" if self.plot_control.current_metric_id() == "spectrogram" else ""
+
+    def _sync_ref_lines(self):
+        """Push the current reference-line set to both the list view and the plot."""
+        self.ref_line_control.set_lines(self.ref_lines)
+        self.visualization_widget.set_reference_lines(self.ref_lines)
+
+    def on_view_changed(self):
+        """Called when a view-scale toggle (lin/log) changes. Recompute-free redraw."""
+        self.logger.info("View scale changed via GUI")
+        self._refresh_view()
+
+    def on_plot_refresh_requested(self):
+        """Called when manual plot refresh is requested."""
+        self.logger.info("Manual plot refresh requested via GUI")
+        self._refresh_view()
+
+    def on_metric_compute_started(self, file_path: str, metric_id: str):
+        """Called when an off-thread metric compute starts."""
+        if file_path not in self._overlay_paths():
+            return  # not in the drawn set; status bar shouldn't lie
+        metric = METRICS.get(metric_id)
+        display = metric.display_name if metric else metric_id
+        self.visualization_widget.set_status(f"Computing {display}...")
+
+    def on_metric_ready(self, file_path: str, metric_id: str):
+        """Called when metric data is available (cached hit or async finish)."""
+        if metric_id != self.plot_control.current_metric_id():
+            return  # user already switched to a different metric
+        if file_path not in self._overlay_paths():
+            return  # no longer part of the overlay set
+        self._refresh_view()
+
+    def on_metric_timing(self, file_path: str, metric_id: str, seconds: float):
+        """An on-demand metric compute finished — report how long it took."""
+        if file_path not in self._overlay_paths():
+            return
+        if metric_id != self.plot_control.current_metric_id():
+            return
+        metric = METRICS.get(metric_id)
+        display = metric.display_name if metric else metric_id
+        self.visualization_widget.set_status(f"{display} computed in {seconds:.1f}s")
+
+    def on_metric_compute_error(self, file_path: str, metric_id: str, error_message: str):
+        self.logger.error(f"Metric compute failed ({metric_id} / {os.path.basename(file_path)}): {error_message}")
+        if file_path in self._overlay_paths():
+            self.visualization_widget.set_status(f"Error computing {metric_id}: {error_message}")
+
+    def _current_file_path(self):
+        item = self.file_list.currentItem()
+        return item.data(Qt.UserRole) if item else None
+
+    def _item_for_path(self, file_path):
+        for i in range(self.file_list.count()):
+            item = self.file_list.item(i)
+            if item.data(Qt.UserRole) == file_path:
+                return item
+        return None
+
+    def _row_index(self, file_path) -> int:
+        for i in range(self.file_list.count()):
+            if self.file_list.item(i).data(Qt.UserRole) == file_path:
+                return i
+        return 0
+
+    def _overlay_paths(self):
+        """File paths whose checkbox is ticked — the set drawn on the graph."""
+        return [
+            self.file_list.item(i).data(Qt.UserRole)
+            for i in range(self.file_list.count())
+            if self.file_list.item(i).checkState() == Qt.Checked
+        ]
+
+    def _refresh_view(self):
+        """Redraw the checked overlay set for the current metric and view-state.
+
+        Renders every dataset whose data is cached; for any that isn't, kicks off
+        an async compute (or a full load if the file was never analysed) and
+        leaves a status note. `on_metric_ready` calls back here when each lands.
+        """
+        paths = self._overlay_paths()
+        metric_id = self.plot_control.current_metric_id()
+        view = self.plot_control.current_view_state()
+        metric = METRICS.get(metric_id)
+        if not paths or metric is None:
+            self.visualization_widget.show_specs([])
+            return
+
+        specs = []
+        pending = 0
+        for path in paths:
+            data = self.analysis_manager.get_metric_data(path, metric_id)
+            if data is None:
+                if self.analysis_manager.is_file_analyzed(path):
+                    self.analysis_manager.request_metric(path, metric_id)
+                else:
+                    self.analysis_manager.analyze_file(path, metric_id)
+                pending += 1
+                continue
+            label = self.analysis_manager.display_label(path)
+            # Colour is keyed to the file's row, not its position in the overlay
+            # subset, so a song keeps its colour as others are ticked/unticked.
+            color = dataset_color(self._row_index(path))
+            spec = apply_x_mode(metric.build_spec(data, view), view.x_mode)
+            specs.append((label, spec, color))
+
+        if specs:
+            self.visualization_widget.show_specs(specs, view)
+        if pending:
+            self.visualization_widget.set_status(
+                f"Computing {metric.display_name} for {pending} file(s)..."
+            )
+
+
+def main():
+    # Parse logging arguments before creating QApplication
+    log_level, log_to_file = parse_log_args()
+
+    # Initialize logging
+    logger = setup_logging(log_level, log_to_file)
+    logger.info("Starting Audio Mastering Analysis Toolkit")
+    logger.info(f"Command line args: log-level={log_level}, log-file={log_to_file}")
+
+    app = QApplication(sys.argv)
+
+    # Initialize font system (loads any fonts/ files, configures fallbacks) then
+    # lock the UI font. M PLUS 1 Code has full Japanese coverage, so this stays
+    # CJK-safe; falls back to the system default if the family isn't present.
+    initialize_fonts()
+    chosen = apply_fixed_font("M PLUS 1 Code", 10)
+    logger.info(f"UI font locked to '{chosen}' at 10pt")
+
+    # Set application style
+    app.setStyle('Fusion')  # Modern cross-platform style
+    logger.debug("Application style set to Fusion")
+
+    window = MainWindow()
+    window.show()
+    logger.info("GUI window displayed")
+
+    sys.exit(app.exec_())
+
+
+if __name__ == '__main__':
     main()
\ No newline at end of file
diff --git a/master_core.py b/master_core.py
index c745792..e40f4f2 100644
--- a/master_core.py
+++ b/master_core.py
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
 import os
 
 import librosa
diff --git a/metrics.py b/metrics.py
index c26088c..d5d50da 100644
--- a/metrics.py
+++ b/metrics.py
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
 """
 Pluggable analysis metrics.
 
diff --git a/plot_control_widget.py b/plot_control_widget.py
index 30ab5d2..a1873ab 100644
--- a/plot_control_widget.py
+++ b/plot_control_widget.py
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
 """
 Plot control widget: pick the metric, set axis scale/mode, refresh the plot.
 
diff --git a/plotspec.py b/plotspec.py
index b66ca0a..a3cbbab 100644
--- a/plotspec.py
+++ b/plotspec.py
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
 """
 Backend-agnostic plot descriptors.
 
diff --git a/pyproject.toml b/pyproject.toml
index 21bc906..554a693 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -19,6 +19,8 @@ dependencies = [
     "pyqtgraph>=0.14.0",
 ]
 
+license = {text = "Apache-2.0"}
+
 [project.scripts]
 ujm = "main:main"
 
diff --git a/ref_line_widget.py b/ref_line_widget.py
index 6a54b5b..151e4ed 100644
--- a/ref_line_widget.py
+++ b/ref_line_widget.py
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
 """
 Reference-line management: a side-panel list of custom horizontal markers plus a
 properties dialog.
diff --git a/setup_fonts.py b/setup_fonts.py
index 0b65a77..19320a4 100644
--- a/setup_fonts.py
+++ b/setup_fonts.py
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
 #!/usr/bin/env python3
 """
 Font setup utility for CJK character support.