- Add main menu UI image and background music (with updated track metadata)
- Rename puzzle assets to include `.png` extensions for consistency
- Introduce `thumbnails.json` manifest to decouple thumbnail paths from puzzle data
- Update `NewPuzzleScene.js` to load thumbnails explicitly via manifest instead of inferring paths
- Update `PuzzleScene.js` to ensure full puzzle images are loaded fresh on initial entry (not during network joins or restores)
