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>

lib/audio/player.ts

@@ -114,18 +114,25 @@ export class AudioPlayer {
   }
 
   /**
-   * Seek to a specific time and start playback
+   * Seek to a specific time
+   * @param time - Time in seconds to seek to
+   * @param autoPlay - Whether to automatically start playback after seeking (default: false)
    */
-  async seek(time: number): Promise<void> {
+  async seek(time: number, autoPlay: boolean = false): Promise<void> {
     if (!this.audioBuffer) return;
 
+    const wasPlaying = this.isPlaying;
     const clampedTime = Math.max(0, Math.min(time, this.audioBuffer.duration));
 
     this.stop();
     this.pauseTime = clampedTime;
 
-    // Always start playback after seeking
-    await this.play(clampedTime);
+    // Auto-play if requested, or continue playing if was already playing
+    if (autoPlay || wasPlaying) {
+      await this.play(clampedTime);
+    } else {
+      this.isPaused = true;
+    }
   }
 
   /**