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>

backend/src/db/migrate.js

@@ -862,7 +862,8 @@ db.exec(`
     ('feature_knowledge_map_enabled', '1'),
     ('feature_board_enabled',         '1'),
     ('feature_biochem_enabled',       '1'),
-    ('feature_live_quiz_enabled',     '1');
+    ('feature_live_quiz_enabled',     '1'),
+    ('feature_classroom_enabled',     '1');
 `);
 
 /* ── Performance indexes ───────────────────────────────────────────────── */
@@ -2773,8 +2774,9 @@ db.exec(`
     UNIQUE(session_id, page_num)
   )
 `);
-// Add template column to existing classroom_pages tables (idempotent)
+// Add template + name columns to existing classroom_pages tables (idempotent)
 try { db.exec(`ALTER TABLE classroom_pages ADD COLUMN template TEXT NOT NULL DEFAULT 'blank'`); } catch (_) { /* already exists */ }
+try { db.exec(`ALTER TABLE classroom_pages ADD COLUMN name TEXT`); } catch (_) { /* already exists */ }
 
 db.exec(`
   CREATE TABLE IF NOT EXISTS classroom_strokes (
@@ -2854,6 +2856,9 @@ db.exec(`
   )
 `);
 
+// Guest token for classroom sessions (public read-only whiteboard access)
+try { db.exec("ALTER TABLE classroom_sessions ADD COLUMN guest_token TEXT UNIQUE"); } catch {}
+
 // Persistent draw permissions (survives server restart)
 db.exec(`
   CREATE TABLE IF NOT EXISTS classroom_draw_permissions (