feat: user preferences sync — server-side storage, whiteboard defaults, dashboard widget visibility

- New table `user_preferences` (user_id PK, JSON blob, updated_at)
- GET/PATCH/DELETE /api/preferences with deep-merge UPSERT
- LS.prefs singleton in api.js: dot-notation get/set, debounced flush (1.5s), server sync
- classroom.html: load wb.color/width/lineStyle/theme from prefs on init; save on change
- dashboard.html: widget configurator panel (gear button) — toggle visibility per-user, persisted server-side

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/server.js

@@ -143,6 +143,7 @@ app.use('/api/bookmarks',    bookmarkRoutes);
 app.use('/api/search',       searchRoutes);
 app.use('/api/flashcards',   flashcardRoutes);
 app.use('/api/settings',    settingsRoutes);
+app.use('/api/preferences', require('./routes/preferences'));
 app.use('/api/analytics',   analyticsRoutes);
 app.use('/api/live',              liveRoutes);
 app.use('/api/classroom/guest',   guestClassroomRoutes); // public — MUST be before /api/classroom