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-biomes.html

@@ -167,6 +167,7 @@
       <div class="bio-topbar">
         <a href="/red-book.html" style="color:var(--rb-muted);text-decoration:none;font-size:12px;border:1px solid var(--rb-border);padding:6px 12px;border-radius:8px;"><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="M17 14 12 3 7 14"/><path d="M4 20 8 11h8l4 9"/><line x1="12" y1="20" x2="12" y2="22"/></svg> Биомы Беларуси</h1>
         <div class="biome-tabs" id="biome-tabs"></div>
         <button id="sound-btn" onclick="toggleSound()" title="Звуки биома" style="display:inline-flex;align-items:center;gap:6px;background:transparent;border:1px solid var(--rb-border);color:var(--rb-muted);font-size:12px;font-weight:600;padding:6px 12px;border-radius:8px;cursor:pointer;flex-shrink:0;transition:all .2s;"><svg class="ic" viewBox="0 0 24 24"><polygon points="11 5 6 9 2 9 2 15 6 15 11 19 11 5"/><line x1="23" y1="9" x2="17" y2="15"/><line x1="17" y1="9" x2="23" y2="15"/></svg> Звук</button>