- Introduce `Sounds.js` utility and load new audio assets (dice, cards, chips, casino).
- Play appropriate sound effects in Backgammon, Blackjack, Holdem, Parchisi, Phase10, SkipBo, and Yatzi.
- Add `background-casino.png` and `background-room.png` to PreloadScene.
- Update OpponentSelectScene with new backgrounds, styled title/subtitle pills, and improved Start/Back buttons.
- Enhance opponent selection with gradient backgrounds and shadows via `applyOpponentStyle`.
- Implement `highlightTile` for better visual feedback in option selectors.
- Randomly select a card back for card games in OpponentSelectScene.
