Changes JamTrack player to use inline styles matching the backing track
player's simpler approach while keeping the macOS window chrome.
- Removed CSS file import, switched to inline styles
- Matched backing track player's control layout
- Circular play (36px) and stop (36px) buttons
- Time + scrubber + time in one row matching backing track
- Mix dropdown and "create custom mix" link below controls
- Close button at bottom centered
- All spacing and colors match backing track player
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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