feat: implement Phase 7.1-7.2 multi-track infrastructure

Added core multi-track support with track management and controls:

**Track Types & Utilities:**
- Track interface with audio buffer, controls (volume/pan/solo/mute)
- Track utility functions for creation, mixing, and gain calculation
- Track color system with 9 preset colors
- Configurable track heights (60-300px)

**Components:**
- TrackHeader: Collapsible track controls with inline name editing
  - Solo/Mute buttons with visual feedback
  - Volume slider (0-100%) and Pan control (L-C-R)
  - Track color indicator and remove button
- Track: Waveform display component with canvas rendering
  - Click-to-seek on waveform
  - Playhead visualization
  - Support for collapsed state
- TrackList: Container managing multiple tracks
  - Scrollable track list with custom scrollbar
  - Add track button
  - Empty state UI

**State Management:**
- useMultiTrack hook with localStorage persistence
- Add/remove/update/reorder track operations
- Track buffer management

Features implemented:
- ✅ Track creation and removal
- ✅ Track naming (editable)
- ✅ Track colors
- ✅ Solo/Mute per track
- ✅ Volume fader per track (0-100%)
- ✅ Pan control per track (L-C-R)
- ✅ Track collapse/expand
- ✅ Track height configuration
- ✅ Waveform visualization per track
- ✅ Multi-track audio mixing utilities

Next: Integrate into AudioEditor and implement multi-track playback

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

types/track.ts

@@ -0,0 +1,55 @@
+/**
+ * Multi-track types and interfaces
+ */
+
+export interface Track {
+  id: string;
+  name: string;
+  color: string;
+  height: number;
+  audioBuffer: AudioBuffer | null;
+
+  // Track controls
+  volume: number; // 0-1
+  pan: number; // -1 (left) to 1 (right)
+  mute: boolean;
+  solo: boolean;
+  recordEnabled: boolean;
+
+  // UI state
+  collapsed: boolean;
+  selected: boolean;
+}
+
+export interface TrackState {
+  tracks: Track[];
+  selectedTrackId: string | null;
+  soloedTracks: Set<string>;
+}
+
+export type TrackColor =
+  | 'red'
+  | 'orange'
+  | 'yellow'
+  | 'green'
+  | 'blue'
+  | 'indigo'
+  | 'purple'
+  | 'pink'
+  | 'gray';
+
+export const TRACK_COLORS: Record<TrackColor, string> = {
+  red: 'rgb(239, 68, 68)',
+  orange: 'rgb(249, 115, 22)',
+  yellow: 'rgb(234, 179, 8)',
+  green: 'rgb(34, 197, 94)',
+  blue: 'rgb(59, 130, 246)',
+  indigo: 'rgb(99, 102, 241)',
+  purple: 'rgb(168, 85, 247)',
+  pink: 'rgb(236, 72, 153)',
+  gray: 'rgb(156, 163, 175)',
+};
+
+export const DEFAULT_TRACK_HEIGHT = 120;
+export const MIN_TRACK_HEIGHT = 60;
+export const MAX_TRACK_HEIGHT = 300;