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/projectile.js

@@ -88,6 +88,11 @@ class ProjectileSim {
     this._cw = w; this._ch = h;
   }
 
+  getParams() {
+    return { v0: this.v0, angle: this.angle, h0: this.h0, g: this.g,
+             drag: this.drag, Cd: this.Cd, mass: this.mass, wind: this.wind,
+             bounce: this.bounce, restitution: this.restitution };
+  }
   setParams({ v0, angle, h0, g, drag, Cd, mass, wind, bounce, restitution } = {}) {
     if (v0          !== undefined) this.v0          = +v0;
     if (angle       !== undefined) this.angle       = +angle;