feat: refactor to multi-track only editor with sidebar controls

Major refactor to simplify the editor and focus exclusively on multi-track editing:

**AudioEditor Changes:**
- Removed single-file waveform view and useAudioPlayer
- Removed all single-file editing operations (cut, copy, paste, trim)
- Simplified to multi-track only with track selection support
- Added selectedTrackId state for track-specific effect chain
- Reduced from ~1500 lines to ~290 lines

**SidePanel Changes:**
- Complete rewrite with 2 tabs: Tracks and Effect Chain
- Tracks tab shows all tracks with inline controls (volume, pan, solo, mute)
- Click tracks to select/deselect
- Effect Chain tab shows effects for selected track
- Removed old file/history/info/effects tabs
- Sidebar now wider (320px) to accommodate inline track controls

**TrackList/Track Changes:**
- Added track selection support (isSelected/onSelect props)
- Visual feedback with ring border when track is selected
- Click anywhere on track to select it

**Workflow:**
1. Import or add audio tracks
2. Select a track in the sidebar or main view
3. Apply effects to selected track via Effect Chain tab
4. Adjust track controls (volume, pan, solo, mute) in Tracks tab

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

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

components/tracks/TrackList.tsx

@@ -12,6 +12,8 @@ export interface TrackListProps {
   zoom: number;
   currentTime: number;
   duration: number;
+  selectedTrackId?: string | null;
+  onSelectTrack?: (trackId: string | null) => void;
   onAddTrack: () => void;
   onImportTrack?: (buffer: AudioBuffer, name: string) => void;
   onRemoveTrack: (trackId: string) => void;
@@ -24,6 +26,8 @@ export function TrackList({
   zoom,
   currentTime,
   duration,
+  selectedTrackId,
+  onSelectTrack,
   onAddTrack,
   onImportTrack,
   onRemoveTrack,
@@ -78,6 +82,8 @@ export function TrackList({
             zoom={zoom}
             currentTime={currentTime}
             duration={duration}
+            isSelected={selectedTrackId === track.id}
+            onSelect={onSelectTrack ? () => onSelectTrack(track.id) : undefined}
             onToggleMute={() =>
               onUpdateTrack(track.id, { mute: !track.mute })
             }