fix: playback position animation frame not updating

Fixed issue where currentTime wasn't updating during playback:
- Removed 'isPlaying' from updatePlaybackPosition dependencies
- This was causing the RAF loop to stop when state changed
- Now animation frame continues running throughout playback
- Playhead now updates smoothly in waveform and timeline slider

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

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

lib/hooks/useMultiTrackPlayer.ts

@@ -34,7 +34,7 @@ export function useMultiTrackPlayer(tracks: Track[]) {
   }, [tracks]);
 
   const updatePlaybackPosition = useCallback(() => {
-    if (!audioContextRef.current || !isPlaying) return;
+    if (!audioContextRef.current) return;
 
     const elapsed = audioContextRef.current.currentTime - startTimeRef.current;
     const newTime = pausedAtRef.current + elapsed;
@@ -43,12 +43,16 @@ export function useMultiTrackPlayer(tracks: Track[]) {
       setIsPlaying(false);
       setCurrentTime(0);
       pausedAtRef.current = 0;
+      if (animationFrameRef.current) {
+        cancelAnimationFrame(animationFrameRef.current);
+        animationFrameRef.current = null;
+      }
       return;
     }
 
     setCurrentTime(newTime);
     animationFrameRef.current = requestAnimationFrame(updatePlaybackPosition);
-  }, [isPlaying, duration]);
+  }, [duration]);
 
   const play = useCallback(() => {
     if (tracks.length === 0 || tracks.every(t => !t.audioBuffer)) return;