feat: implement real-time playback level monitoring for all tracks

Added comprehensive playback level monitoring system that shows
real-time audio levels during playback for each track.

useMultiTrackPlayer Hook:
- Added AnalyserNode for each track in audio graph
- Implemented RMS-based level calculation with requestAnimationFrame
- Added trackLevels state (Record<string, number>) tracking levels by track ID
- Insert analysers after effects chain, before master gain
- Monitor levels continuously during playback
- Clean up level monitoring on pause/stop

Audio Graph Chain:
source -> gain -> pan -> effects -> analyser -> master gain -> destination

AudioEditor Integration:
- Extract trackLevels from useMultiTrackPlayer hook
- Pass trackLevels down to TrackList component

TrackList & Track Components:
- Accept and forward trackLevels prop
- Pass playbackLevel to individual Track components
- Track component displays appropriate level:
  * Recording level (with "Input" label) when armed/recording
  * Playback level (with "Level" label) during normal playback

Visual Feedback:
- Color-coded meters: green -> yellow (70%) -> red (90%)
- Real-time percentage display
- Seamless switching between input and output modes

This completes Phase 8 (Recording) with full bidirectional level monitoring!

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

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

components/tracks/TrackList.tsx

@@ -24,6 +24,7 @@ export interface TrackListProps {
   onToggleRecordEnable?: (trackId: string) => void;
   recordingTrackId?: string | null;
   recordingLevel?: number;
+  trackLevels?: Record<string, number>;
 }
 
 export function TrackList({
@@ -42,6 +43,7 @@ export function TrackList({
   onToggleRecordEnable,
   recordingTrackId,
   recordingLevel = 0,
+  trackLevels = {},
 }: TrackListProps) {
   const [importDialogOpen, setImportDialogOpen] = React.useState(false);
 
@@ -164,6 +166,7 @@ export function TrackList({
             }
             isRecording={recordingTrackId === track.id}
             recordingLevel={recordingTrackId === track.id ? recordingLevel : 0}
+            playbackLevel={trackLevels[track.id] || 0}
           />
         ))}
       </div>