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/gradebook.html

@@ -215,6 +215,7 @@
       <a href="/knowledge-map" class="sb-link"><i data-lucide="share-2"          class="sb-icon"></i><span class="sb-lbl">Карта знаний</span></a>
       <a href="/red-book.html" class="sb-link"><i data-lucide="leaf"             class="sb-icon"></i><span class="sb-lbl">Красная книга</span></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>
       <div class="sb-divider"></div>
       <a href="/analytics"     class="sb-link"><i data-lucide="bar-chart-2"      class="sb-icon"></i><span class="sb-lbl">Аналитика</span></a>
       <a href="/question-bank" class="sb-link"><i data-lucide="database"         class="sb-icon"></i><span class="sb-lbl">Банк вопросов</span></a>