feat: WebSocket real-time + rAF render gate + guest board + screen picker

Classroom performance:
- WebSocket server (ws-server.js) for low-latency cursor & stroke preview
  Replaces HTTP POST per event → eliminates per-message auth overhead
  Session member cache (30s TTL) avoids SQLite query per WS message
  Fallback to HTTP POST when WS not connected
- Cursor throttle reduced 100ms → 33ms (~30fps)
- Stroke preview throttle reduced 50ms → 20ms
- whiteboard.js: render() is now rAF-gated (_doRender/_rafPending)
  Multiple render() calls within one frame collapse into one repaint
  document.hidden check — zero CPU when tab is in background
  visibilitychange listener restores canvas on tab focus

Guest board:
- guestClassroom.js route: public token-based read-only access
- guest-board.html: name entry + read-only whiteboard + SSE
- SSE: addGuestClient/removeGuestClient/emitToGuests

Screen share picker:
- Discord-style modal with tab switching (screen/window/tab)
- Live video preview before confirming share
- useExistingScreenStream() in ClassroomRTC

Fullscreen exit overlay:
- #cr-fs-exit-overlay button inside cr-board-wrap
- Visible only via CSS :fullscreen selector (touchpad users)

File sharing from library:
- Teacher picks file from library, sends as styled card in chat
- crDownloadLibraryFile() fetches with Bearer auth

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/controllers/adminController.js

@@ -324,7 +324,7 @@ function getFeatures(_req, res) {
 /* ── PATCH /api/admin/features ──────────────────────────────────────── */
 function updateFeatures(req, res) {
   const allowed = ['crossword', 'hangman', 'pet', 'red_book', 'collection',
-                   'flashcards', 'knowledge_map', 'board', 'biochem', 'live_quiz'];
+                   'flashcards', 'knowledge_map', 'board', 'biochem', 'live_quiz', 'classroom'];
   const updates = req.body;
   const stmt = db.prepare("INSERT OR REPLACE INTO app_settings (key, value) VALUES (?, ?)");
   const changed = [];