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/Track.tsx

@@ -35,6 +35,7 @@ export interface TrackProps {
   onToggleRecordEnable?: () => void;
   isRecording?: boolean;
   recordingLevel?: number;
+  playbackLevel?: number;
 }
 
 export function Track({
@@ -61,6 +62,7 @@ export function Track({
   onToggleRecordEnable,
   isRecording = false,
   recordingLevel = 0,
+  playbackLevel = 0,
 }: TrackProps) {
   const canvasRef = React.useRef<HTMLCanvasElement>(null);
   const containerRef = React.useRef<HTMLDivElement>(null);
@@ -564,12 +566,12 @@ export function Track({
               </label>
               <div className="flex-1">
                 <InputLevelMeter
-                  level={track.recordEnabled || isRecording ? recordingLevel : 0}
+                  level={track.recordEnabled || isRecording ? recordingLevel : playbackLevel}
                   orientation="horizontal"
                 />
               </div>
               <span className="text-xs text-muted-foreground w-10 text-right flex-shrink-0">
-                {Math.round((track.recordEnabled || isRecording ? recordingLevel : 0) * 100)}%
+                {Math.round((track.recordEnabled || isRecording ? recordingLevel : playbackLevel) * 100)}%
               </span>
             </div>
           </>