Issue 1 - End of track playback:
- When track ends, call SessionStopPlay() and reset position to 0
- Ensures clean state for next playback
- Fixes "3 click" bug when restarting after track ends
Issue 2 & 3 - Volume and loop not working in popup mode:
- Add getBackingTrackPath() helper to handle both formats
- In popup mode, backingTrack is a string (path)
- In modal mode, backingTrack is an object with .path property
- Update handleLoopChange to use helper
- Fixes "No backing track available" error
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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