- Import and register new `BootScene` and `MenuScene` in `main.js`.
- Create `BootScene` to generate all shared textures once at startup and then start the menu.
- Implement full-featured `MenuScene` with background asteroids, wandering alien, title, controls panel, and start button (including ENTER key support).
- Remove texture generation logic and helper from `GameScene`; it now relies on textures created by `BootScene`.
- Change game‑over handling to return to the menu instead of restarting the game.
