refactor(decnet_web/MazeNET): extract useFullscreenMode

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.

decnet_web/src/components/MazeNET/useFullscreenMode.ts

@@ -0,0 +1,62 @@
+import { useEffect, useState } from 'react';
+
+const BODY_CLASS = 'maze-fullscreen';
+
+export interface UseFullscreenModeResult {
+  fullscreen: boolean;
+  setFullscreen: (next: boolean | ((prev: boolean) => boolean)) => void;
+  toggle: () => void;
+}
+
+/** Fullscreen-mode state for the MazeNET canvas. Owns four
+ *  side-effects:
+ *    1. Toggle a body class so the page CSS can hide its chrome.
+ *    2. Request/exit the browser-level fullscreen API (failures
+ *       are ignored; chrome-only mode still works without it).
+ *    3. Listen for fullscreenchange so F11/Esc from outside our
+ *       button keeps internal state in sync.
+ *    4. Esc shortcut to leave fullscreen via keyboard. */
+export function useFullscreenMode(): UseFullscreenModeResult {
+  const [fullscreen, setFullscreen] = useState(false);
+
+  useEffect(() => {
+    if (fullscreen) document.body.classList.add(BODY_CLASS);
+    else document.body.classList.remove(BODY_CLASS);
+    return () => document.body.classList.remove(BODY_CLASS);
+  }, [fullscreen]);
+
+  // Request/exit browser fullscreen alongside the in-app chrome hide.
+  // Ignore failures (fullscreen requires a user gesture; the chrome-only
+  // mode still works if the API rejects).
+  useEffect(() => {
+    if (fullscreen && !document.fullscreenElement) {
+      document.documentElement.requestFullscreen?.().catch(() => {});
+    } else if (!fullscreen && document.fullscreenElement) {
+      document.exitFullscreen?.().catch(() => {});
+    }
+  }, [fullscreen]);
+
+  // Sync state if the user presses F11/Esc to leave fullscreen from
+  // outside our button.
+  useEffect(() => {
+    const onFsChange = () => {
+      if (!document.fullscreenElement) setFullscreen(false);
+    };
+    document.addEventListener('fullscreenchange', onFsChange);
+    return () => document.removeEventListener('fullscreenchange', onFsChange);
+  }, []);
+
+  useEffect(() => {
+    const onKey = (e: KeyboardEvent) => {
+      if (e.key === 'Escape' && fullscreen) setFullscreen(false);
+    };
+    window.addEventListener('keydown', onKey);
+    return () => window.removeEventListener('keydown', onKey);
+  }, [fullscreen]);
+
+  return {
+    fullscreen,
+    setFullscreen,
+    toggle: () => setFullscreen((f) => !f),
+  };
+}