feat: implement global volume and mute controls

- Add masterVolume state to AudioEditor (default 0.8)
- Pass masterVolume to useMultiTrackPlayer hook
- Create master gain node in audio graph
- Connect all tracks through master gain before destination
- Update master gain in real-time when volume changes
- Wire up PlaybackControls volume slider and mute button
- Clean up master gain node on unmount

Fixes global volume and mute controls not working in transport controls.

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

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

lib/hooks/useMultiTrackPlayer.ts

@@ -9,7 +9,7 @@ export interface MultiTrackPlayerState {
   duration: number;
 }
 
-export function useMultiTrackPlayer(tracks: Track[]) {
+export function useMultiTrackPlayer(tracks: Track[], masterVolume: number = 1) {
   const [isPlaying, setIsPlaying] = useState(false);
   const [currentTime, setCurrentTime] = useState(0);
   const [duration, setDuration] = useState(0);
@@ -18,6 +18,7 @@ export function useMultiTrackPlayer(tracks: Track[]) {
   const sourceNodesRef = useRef<AudioBufferSourceNode[]>([]);
   const gainNodesRef = useRef<GainNode[]>([]);
   const panNodesRef = useRef<StereoPannerNode[]>([]);
+  const masterGainNodeRef = useRef<GainNode | null>(null);
   const startTimeRef = useRef<number>(0);
   const pausedAtRef = useRef<number>(0);
   const animationFrameRef = useRef<number | null>(null);
@@ -71,11 +72,20 @@ export function useMultiTrackPlayer(tracks: Track[]) {
     });
     gainNodesRef.current.forEach(node => node.disconnect());
     panNodesRef.current.forEach(node => node.disconnect());
+    if (masterGainNodeRef.current) {
+      masterGainNodeRef.current.disconnect();
+    }
 
     sourceNodesRef.current = [];
     gainNodesRef.current = [];
     panNodesRef.current = [];
 
+    // Create master gain node
+    const masterGain = audioContext.createGain();
+    masterGain.gain.setValueAtTime(masterVolume, audioContext.currentTime);
+    masterGain.connect(audioContext.destination);
+    masterGainNodeRef.current = masterGain;
+
     // Create audio graph for each track
     for (const track of tracks) {
       if (!track.audioBuffer) continue;
@@ -93,10 +103,10 @@ export function useMultiTrackPlayer(tracks: Track[]) {
       // Set pan
       panNode.pan.setValueAtTime(track.pan, audioContext.currentTime);
 
-      // Connect: source -> gain -> pan -> destination
+      // Connect: source -> gain -> pan -> master gain -> destination
       source.connect(gainNode);
       gainNode.connect(panNode);
-      panNode.connect(audioContext.destination);
+      panNode.connect(masterGain);
 
       // Start playback from current position
       source.start(0, pausedAtRef.current);
@@ -119,7 +129,7 @@ export function useMultiTrackPlayer(tracks: Track[]) {
     startTimeRef.current = audioContext.currentTime;
     setIsPlaying(true);
     updatePlaybackPosition();
-  }, [tracks, duration, updatePlaybackPosition]);
+  }, [tracks, duration, masterVolume, updatePlaybackPosition]);
 
   const pause = useCallback(() => {
     if (!audioContextRef.current || !isPlaying) return;
@@ -200,6 +210,16 @@ export function useMultiTrackPlayer(tracks: Track[]) {
     });
   }, [tracks, isPlaying]);
 
+  // Update master volume when it changes
+  useEffect(() => {
+    if (!isPlaying || !audioContextRef.current || !masterGainNodeRef.current) return;
+
+    masterGainNodeRef.current.gain.setValueAtTime(
+      masterVolume,
+      audioContextRef.current.currentTime
+    );
+  }, [masterVolume, isPlaying]);
+
   // Cleanup on unmount
   useEffect(() => {
     return () => {
@@ -216,6 +236,9 @@ export function useMultiTrackPlayer(tracks: Track[]) {
       });
       gainNodesRef.current.forEach(node => node.disconnect());
       panNodesRef.current.forEach(node => node.disconnect());
+      if (masterGainNodeRef.current) {
+        masterGainNodeRef.current.disconnect();
+      }
     };
   }, []);