- Introduce `index.html` implementing the full game UI, neon‑styled CSS, canvas‑based Tetris logic, particle explosions, level‑up animation, background scanlines/effects, and music/FX handling.
- Add audio assets: sound effects (`place.mp3`, `explode.mp3`, `levelUp.mp3`) and four background music tracks (`track01‑04.mp3`).
- Wire up music playlist, level‑up track change, and UI updates for score, level, and lines.
