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/chemsandbox.js

@@ -1,4 +1,16 @@
 'use strict';
+
+/* Strip SVG markup for canvas fillText — replaces icon SVGs with Unicode */
+function _csClean(s) {
+  if (!s || !s.includes('<svg')) return s;
+  return s.replace(/<svg[\s\S]*?<\/svg>/g, m => {
+    if (m.includes('x1="5" y1="12" x2="19"'))  return '\u2192'; // → right arrow
+    if (m.includes('x1="12" y1="5" x2="12" y2="19"')) return '\u2193'; // ↓ down (precip)
+    if (m.includes('x1="12" y1="19" x2="12" y2="5"')) return '\u2191'; // ↑ up (gas)
+    return '';
+  });
+}
+
 /* ════════════════════════════════════════════════════════════════
    ChemSandboxSim v2 — «Химическая песочница»
    • Колба Эрленмейера с реалистичным стеклом
@@ -1089,7 +1101,7 @@ class ChemSandboxSim {
     // ── Молекулярное уравнение ──
     ctx.font = 'bold 17px "JetBrains Mono", monospace';
     ctx.fillStyle = rx.fx.none ? 'rgba(255,100,100,0.75)' : 'rgba(255,255,255,0.95)';
-    ctx.fillText(rx.eq, W / 2, y);
+    ctx.fillText(_csClean(rx.eq), W / 2, y);
     y += 22;
 
     // ── Тип реакции + пояснение ──
@@ -1109,7 +1121,7 @@ class ChemSandboxSim {
     if (rx.why) {
       ctx.font = '13px sans-serif';
       ctx.fillStyle = 'rgba(255,255,255,0.55)';
-      ctx.fillText(rx.why, W / 2, y);
+      ctx.fillText(_csClean(rx.why), W / 2, y);
       y += 17;
     }
 
@@ -1117,7 +1129,7 @@ class ChemSandboxSim {
     if (rx.ionFull) {
       ctx.font = '13px "JetBrains Mono", monospace';
       ctx.fillStyle = 'rgba(155,200,255,0.60)';
-      ctx.fillText('Полн.: ' + rx.ionFull, W / 2, y);
+      ctx.fillText('Полн.: ' + _csClean(rx.ionFull), W / 2, y);
       y += 16;
     }
 
@@ -1125,7 +1137,7 @@ class ChemSandboxSim {
     if (rx.ionNet) {
       ctx.font = 'bold 13px "JetBrains Mono", monospace';
       ctx.fillStyle = 'rgba(123,245,164,0.75)';
-      ctx.fillText('Сокр.: ' + rx.ionNet, W / 2, y);
+      ctx.fillText('Сокр.: ' + _csClean(rx.ionNet), W / 2, y);
       y += 16;
     }