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/core/KGProject.ts

@@ -1,6 +1,7 @@
 import { Expose, Type } from 'class-transformer';
 import { KGTrack } from './track/KGTrack';
 import { KGMidiTrack } from './track/KGMidiTrack';
+import { KGAudioTrack } from './track/KGAudioTrack';
 import { type TimeSignature, WithDefault } from '../types/projectTypes';
 import { TIME_CONSTANTS, KEY_SIGNATURE_MAP } from '../constants/coreConstants';
 
@@ -47,7 +48,7 @@ export class KGProject {
   @WithDefault(0)
   private projectStructureVersion: number = 0;
 
-  public static readonly CURRENT_PROJECT_STRUCTURE_VERSION: number = 3;
+  public static readonly CURRENT_PROJECT_STRUCTURE_VERSION: number = 4;
   
   @Expose()
   @Type(() => KGTrack, {
@@ -56,6 +57,7 @@ export class KGProject {
       subTypes: [
         { value: KGTrack, name: 'KGTrack' },
         { value: KGMidiTrack, name: 'KGMidiTrack' },
+        { value: KGAudioTrack, name: 'KGAudioTrack' },
       ],
     },
   })