fix: proper seeking behavior with optional auto-play

Complete rewrite of seeking logic to support both scrubbing and
click-to-play functionality with proper state management.

Changes:
1. Added autoPlay parameter to seek() methods across the stack
2. Waveform behavior:
   - Click and drag → scrubs WITHOUT auto-play during drag
   - Mouse up after drag → auto-plays from release position
   - This allows smooth scrubbing while dragging
3. Timeline slider behavior:
   - onChange → seeks WITHOUT auto-play (smooth dragging)
   - onMouseUp/onTouchEnd → auto-plays from final position
4. Transport button state now correctly syncs with playback state

Technical implementation:
- player.seek(time, autoPlay) - autoPlay defaults to false
- If autoPlay=true OR was already playing → continues playback
- If autoPlay=false AND wasn't playing → just seeks (isPaused=true)
- useAudioPlayer.seek() now reads actual player state after seeking

User experience:
✓ Click on waveform → music plays from that position
✓ Drag on waveform → scrubs smoothly, plays on release
✓ Drag timeline slider → scrubs smoothly, plays on release
✓ Transport buttons show correct state (Play/Pause)

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

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

components/editor/PlaybackControls.tsx

@@ -15,7 +15,7 @@ export interface PlaybackControlsProps {
   onPlay: () => void;
   onPause: () => void;
   onStop: () => void;
-  onSeek: (time: number) => void;
+  onSeek: (time: number, autoPlay?: boolean) => void;
   onVolumeChange: (volume: number) => void;
   disabled?: boolean;
   className?: string;
@@ -82,7 +82,9 @@ export function PlaybackControls({
           max={duration || 100}
           step={0.01}
           value={currentTime}
-          onChange={(e) => onSeek(parseFloat(e.target.value))}
+          onChange={(e) => onSeek(parseFloat(e.target.value), false)}
+          onMouseUp={(e) => onSeek(parseFloat((e.target as HTMLInputElement).value), true)}
+          onTouchEnd={(e) => onSeek(parseFloat((e.target as HTMLInputElement).value), true)}
           disabled={disabled || duration === 0}
           className={cn(
             'w-full h-2 bg-secondary rounded-lg appearance-none cursor-pointer',