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>

components/editor/AudioEditor.tsx

@@ -19,6 +19,7 @@ export function AudioEditor() {
   const [importDialogOpen, setImportDialogOpen] = React.useState(false);
   const [selectedTrackId, setSelectedTrackId] = React.useState<string | null>(null);
   const [zoom, setZoom] = React.useState(1);
+  const [masterVolume, setMasterVolume] = React.useState(0.8);
 
   const { addToast } = useToast();
 
@@ -41,7 +42,7 @@ export function AudioEditor() {
     stop,
     seek,
     togglePlayPause,
-  } = useMultiTrackPlayer(tracks);
+  } = useMultiTrackPlayer(tracks, masterVolume);
 
   // Effect chain (for selected track)
   const {
@@ -290,12 +291,12 @@ export function AudioEditor() {
               isPaused={!isPlaying}
               currentTime={currentTime}
               duration={duration}
-              volume={1}
+              volume={masterVolume}
               onPlay={play}
               onPause={pause}
               onStop={stop}
               onSeek={seek}
-              onVolumeChange={() => {}}
+              onVolumeChange={setMasterVolume}
               currentTimeFormatted={formatDuration(currentTime)}
               durationFormatted={formatDuration(duration)}
             />