Per-metric ref lines with Hz spectrogram units; relative-time toggle; fixed font

Reference lines:
- Kept per metric (dict keyed by metric_id) so switching metrics and coming back
  preserves them, instead of clearing on every switch
- Spectrogram lines read/edit/drag in Hz: the renderer installs Hz<->row-index
  transforms (the heatmap y-axis is a row index), so value, label, the edit
  dialog, and the new-line default all speak frequency. Curve metrics stay
  identity. Round-trip verified (1000 Hz -> row -> 1000 Hz)
- Line label and drag-readback always in the metric's natural units

Controls:
- Relative-time abs/rel is now a checkbox toggle, not a dropdown
- Removed the font control panel; UI font is locked to M PLUS 1 Code @ 10pt via
  font_manager.apply_fixed_font (system-default fallback). Deleted
  font_control_widget.py

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ref_line_widget.py

@@ -26,7 +26,7 @@ _STYLE_CHOICES = [("Solid", "solid"), ("Dashed", "dash"), ("Dotted", "dot")]
 class RefLineDialog(QDialog):
   """Edit one reference line's properties. Read the result via `result_props`."""
 
-  def __init__(self, parent, props: RefLineProps):
+  def __init__(self, parent, props: RefLineProps, value_units: str = ""):
     super().__init__(parent)
     self.setWindowTitle("Reference line")
     self._color = props.color
@@ -37,6 +37,8 @@ class RefLineDialog(QDialog):
     self.value_spin.setRange(-1e6, 1e6)
     self.value_spin.setDecimals(2)
     self.value_spin.setValue(props.value)
+    if value_units:
+      self.value_spin.setSuffix(f" {value_units}")
     form.addRow("Value:", self.value_spin)
 
     self.color_button = QPushButton()