Add Full Screen Button for Better UX

### Add a full screen button to the game UI for users who may not know about the F11 keyboard shortcut.

**Rationale:**
While F11 provides full screen functionality, many users aren't aware of this shortcut. A visible button would improve discoverability and user experience.

**Implementation:**
- Add a full screen toggle button to the game interface
- Use browser's Full Screen API (`requestFullscreen()` / `exitFullscreen()`)
- Button should show current state (enter/exit full screen)
- Provide alternative to F11 for better accessibility

**Benefits:**
- Better user experience for those unfamiliar with F11
- More intuitive interface
- Improved accessibility