fix: seeking backwards no longer jumps to frame 0

Fixed critical bug where seeking backwards during playback would
jump to position 0 instead of the desired position.

Root cause:
- When stop() was called on a playing source node, it triggered
  the onended event callback
- The onended callback would set pauseTime = 0 (thinking playback
  naturally ended)
- This interfered with the seek operation which was trying to set
  a new position

Solution:
- Clear sourceNode.onended callback BEFORE calling stop()
- This prevents the ended event from firing when we manually stop
- Seeking now works correctly in all directions

The fix ensures clean state transitions during seeking operations.

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

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

lib/audio/player.ts

@@ -86,6 +86,8 @@ export class AudioPlayer {
   stop(): void {
     if (this.sourceNode) {
       try {
+        // Clear onended callback first to prevent interference
+        this.sourceNode.onended = null;
         this.sourceNode.stop();
       } catch (error) {
         // Ignore errors if already stopped