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/angrybirds.js

@@ -792,7 +792,7 @@ class AngryBirdsSim {
 
     /* Planet + g — second line, readable */
     ctx.font = '13px Manrope,sans-serif'; ctx.fillStyle = 'rgba(255,255,255,0.72)';
-    ctx.fillText(`${pl.label}   g = ${pl.g} м/с²`, 14, 50);
+    ctx.fillText(`${pl.label.replace(/<svg[\s\S]*?<\/svg>/g, '').trim()}   g = ${pl.g} м/с²`, 14, 50);
 
     /* Wind reminder */
     if (lvl?.wind) {