- Introduce `src/audio.js` module for centralized music playback and cleanup
- Add background music tracks for all four games and the main menu
- Refactor game config files to support multiple difficulty levels via `levels` object
- Update game scenes to load level-specific configurations and track progression
- Add win conditions based on level-specific objectives (e.g., bugs to pass, waves cleared)
- Update HUDs to display current progress toward level completion
