Reference-line manager, relative time axis, axis-aware ref-line UX

Reference lines:
- Side-panel cluster (ref_line_widget.py): list of lines with Add/Edit/Remove/
  Clear and a properties dialog (value, colour, line style, tag)
- Lines own their state as RefLineProps (held by MainWindow), drawn with a
  triangle drag-handle; dragging writes the value back and refreshes the list
- Replaces the old centre-only add-and-drag-the-whole-line approach

Comparison:
- Time-axis mode selector: Absolute (seconds) vs Relative (% of each track's own
  length), so a long track and a short one line up by song position. Pure view
  transform (plotspec.apply_x_mode), no recompute
- Overlaid different-length tracks now autorange their x union in absolute mode
  instead of clipping to the first track's span

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

pyproject.toml

@@ -33,6 +33,7 @@ py-modules = [
     "font_manager",
     "font_control_widget",
     "plot_control_widget",
+    "ref_line_widget",
     "logger_setup",
     "setup_fonts",
 ]