feat: add playback speed control (0.25x - 2x)

Implemented variable playback speed functionality:
- Added playbackRate state and ref to useMultiTrackPlayer (0.25x - 2x range)
- Applied playback rate to AudioBufferSourceNode.playbackRate
- Updated timing calculations to account for playback rate
- Real-time playback speed adjustment for active playback
- Dropdown UI control in PlaybackControls with preset speeds
- Integrated changePlaybackRate function through AudioEditor

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

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

components/editor/AudioEditor.tsx

@@ -300,6 +300,8 @@ export function AudioEditor() {
     toggleLoop,
     setLoopPoints,
     setLoopFromSelection,
+    playbackRate,
+    changePlaybackRate,
   } = useMultiTrackPlayer(tracks, masterVolume, handleAutomationRecording);
 
   // Reset latch triggered state when playback stops
@@ -1983,6 +1985,8 @@ export function AudioEditor() {
             loopEnd={loopEnd}
             onToggleLoop={toggleLoop}
             onSetLoopPoints={setLoopPoints}
+            playbackRate={playbackRate}
+            onPlaybackRateChange={changePlaybackRate}
           />
         </div>
       </div>