fix: prevent playhead marker from continuing after pause

Fixed race condition where animation frames could continue after pause() by
adding an isPlayingRef that's checked at the start of updatePlaybackPosition.
This ensures any queued animation frames will exit early if pause was called,
preventing the playhead from continuing to move when audio has stopped.

This fixes the issue where the playhead marker would occasionally keep moving
after hitting spacebar to pause, even though no audio was playing.

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

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

lib/hooks/useMultiTrackPlayer.ts

@@ -63,6 +63,7 @@ export function useMultiTrackPlayer(
   const loopStartRef = useRef<number>(0);
   const loopEndRef = useRef<number>(0);
   const playbackRateRef = useRef<number>(1.0);
+  const isPlayingRef = useRef<boolean>(false);
 
   // Keep tracksRef in sync with tracks prop
   useEffect(() => {
@@ -319,7 +320,7 @@ export function useMultiTrackPlayer(
   }, []);
 
   const updatePlaybackPosition = useCallback(() => {
-    if (!audioContextRef.current) return;
+    if (!audioContextRef.current || !isPlayingRef.current) return;
 
     const elapsed = (audioContextRef.current.currentTime - startTimeRef.current) * playbackRateRef.current;
     const newTime = pausedAtRef.current + elapsed;
@@ -500,6 +501,7 @@ export function useMultiTrackPlayer(
     }
 
     startTimeRef.current = audioContext.currentTime;
+    isPlayingRef.current = true;
     setIsPlaying(true);
     updatePlaybackPosition();
 
@@ -530,6 +532,7 @@ export function useMultiTrackPlayer(
     pausedAtRef.current = Math.min(pausedAtRef.current + elapsed, duration);
     setCurrentTime(pausedAtRef.current);
 
+    isPlayingRef.current = false;
     setIsPlaying(false);
 
     // Stop level monitoring