Lift the four fullscreen-related side-effects off the page shell.
The hook owns:
1. body class toggle so page CSS can hide its chrome
2. browser fullscreen API request/exit (failures ignored)
3. fullscreenchange listener so F11/Esc from outside our button
keeps internal state in sync
4. Esc keystroke handler
Returns { fullscreen, setFullscreen, toggle }.
- New MazeNET/useFullscreenMode.ts
- useFullscreenMode.test.ts (jsdom) covers initial toggle, body
class lifecycle, Esc-to-exit, and unmount cleanup.
- MazeNET.tsx loses ~30 LOC of inline state + effects.
