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>

frontend/js/labs/waves.js

@@ -59,6 +59,10 @@ class WavesSim {
     this._emit();
   }
 
+  getParams() {
+    return { A1: this._A1, f1: this._f1, phi1: this._phi1, A2: this._A2, f2: this._f2, phi2: this._phi2,
+             n: this._n, speed: this._speed, mode: this._mode };
+  }
   setParams({ A1, f1, phi1, A2, f2, phi2, n, speed } = {}) {
     if (A1    !== undefined) this._A1    = Math.max(5,   Math.min(90, +A1));
     if (f1    !== undefined) this._f1    = Math.max(0.3, Math.min(4,  +f1));