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/red-book-ecosystem.html

@@ -170,6 +170,7 @@
       <div class="eco-topbar">
         <a href="/red-book.html" class="eco-back"><svg class="ic" viewBox="0 0 24 24"><line x1="19" y1="12" x2="5" y2="12"/><polyline points="12 19 5 12 12 5"/></svg> Красная книга</a>
       <a href="/classroom" class="sb-link"><i data-lucide="presentation" class="sb-icon"></i><span class="sb-lbl">Онлайн-урок</span></a>
+      <a href="/lesson-history" class="sb-link"><i data-lucide="archive" class="sb-icon"></i><span class="sb-lbl">Архив уроков</span></a>
         <h1><svg class="ic" viewBox="0 0 24 24"><path d="M4.5 16.5c-1.5 1.26-2 5-2 5s3.74-.5 5-2c.71-.84.7-2.13-.09-2.91a2.18 2.18 0 0 0-2.91-.09z"/><path d="m12 15-3-3a22 22 0 0 1 2-3.95A12.88 12.88 0 0 1 22 2c0 2.72-.78 7.5-6 11a22.35 22.35 0 0 1-4 2z"/><path d="M9 12H4s.55-3.03 2-4c1.62-1.08 5 0 5 0"/><path d="M12 15v5s3.03-.55 4-2c1.08-1.62 0-5 0-5"/></svg> Пищевые сети · Симулятор экосистем</h1>
         <span style="margin-left:auto;font-size:12px;color:var(--rb-muted)" id="node-count"></span>
         <button onclick="toggleEnergyFlow()" id="btn-energy" title="Анимация потока энергии" style="background:transparent;border:1px solid var(--rb-border);color:var(--rb-muted);font-size:12px;padding:6px 12px;border-radius:8px;cursor:pointer;transition:all .2s"><svg class="ic" viewBox="0 0 24 24"><polygon points="13 2 3 14 12 14 11 22 21 10 12 10 13 2"/></svg> Поток</button>