feat: add audio track support with WAV/MP3 playback and looping

Introduce KGAudioTrack and KGAudioRegion as new track/region types
alongside existing MIDI tracks. Users can import WAV, MP3, OGG, FLAC,
and AAC files into audio tracks with full playback, looping, and
solo/mute/volume support.

New core models:
- KGAudioTrack (extends KGTrack with TrackType.Wave)
- KGAudioRegion (extends KGRegion with audio file reference)
- KGAudioPlayerBus (Tone.Gain + ToneBufferSource-based playback)
- KGAudioFileStorage (OPFS media/ directory for binary audio files)

New commands:
- AddAudioTrackCommand (create audio track with player bus)
- ImportAudioCommand (import audio file, create region at playhead,
auto-expand maxBars)

Playback features:
- Audio regions scheduled via Tone.Transport alongside MIDI events
- Loop support with duration capping at loop boundaries
- Resume safety offset to prevent ToneBufferSource timing misses
- Cross-track buffer copy when moving audio regions between tracks
- Proper buffer cleanup on track deletion and project switch

UI changes:
- Separate "+ MIDI" and "+ Audio" buttons in top-left spacer
- Audio track shows upload button instead of instrument selector
- FileImportModal reused for audio file drag-and-drop upload
- Real waveform rendering on audio region canvas
- Green color scheme for audio regions (vs blue for MIDI)
- Pencil icon and resize handles hidden for audio regions
- Cross-type region moves blocked (MIDI ↔ Audio)
- Piano roll blocked for audio regions
- Audio region deletion support

Infrastructure:
- Project structure version bumped to 4 (no-op migration)
- class-transformer discriminators updated for new subtypes
- RemoveTrackCommand updated to clean up audio player buses

src/components/track/Region.css

@@ -71,6 +71,28 @@
   position: relative; /* Allow overlayed controls */
 }
 
+.region-content.audio-region-content {
+  background-color: #90EE90; /* Light green for audio regions */
+}
+
+/* Audio region overrides */
+.track-region.audio-region {
+  background-color: #3a6b4a;
+  border-color: #4a8b5a;
+}
+
+.track-region.audio-region:hover {
+  box-shadow: 0 0 0 1px #6aab7a;
+}
+
+.track-region.audio-region .region-header {
+  background-color: #4a8b5a;
+}
+
+.track-region.audio-region:hover .region-header {
+  background-color: #5a9b6a;
+}
+
 /* Region pencil trigger inside content */
 .region-pencil-btn {
   position: absolute;