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/forcesandbox.js

@@ -1787,7 +1787,7 @@ class ForceSandboxSim {
 
       // Угловая скорость ω — фиолетовая метка справа от тела
       if (hasOmg) {
-        const sym  = b.omega > 0 ? '<svg class="ic" viewBox="0 0 24 24"><polyline points="23 4 23 10 17 10"/><path d="M20.49 15a9 9 0 1 1-.49-4.12"/></svg>' : '<svg class="ic" viewBox="0 0 24 24"><polyline points="1 4 1 10 7 10"/><path d="M3.51 15a9 9 0 1 0 .49-4.12"/></svg>';
+        const sym  = b.omega > 0 ? '\u21BB' : '\u21BA';
         const labX = b.x + halfW + 7;
         const labY = b.type === 'box' ? b.y - b.h * 0.12 : b.y - b.r * 0.15;
         ctx.save();
@@ -1852,7 +1852,7 @@ class ForceSandboxSim {
     ctx.fillStyle = '#EF476F';
     ctx.fillText(`p=${(body.mass * spd / S).toFixed(1)} кг·м/с`, cx, cy + r + 12);
     if (Math.abs(body.omega) > 0.05) {
-      const sym = body.omega > 0 ? '<svg class="ic" viewBox="0 0 24 24"><polyline points="23 4 23 10 17 10"/><path d="M20.49 15a9 9 0 1 1-.49-4.12"/></svg>' : '<svg class="ic" viewBox="0 0 24 24"><polyline points="1 4 1 10 7 10"/><path d="M3.51 15a9 9 0 1 0 .49-4.12"/></svg>';
+      const sym = body.omega > 0 ? '\u21BB' : '\u21BA';
       ctx.fillStyle = '#9B5DE5';
       ctx.fillText(`${sym} ω=${Math.abs(body.omega).toFixed(2)} рад/с`, cx, cy + r + 22);
     }