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>
Maxim Dolgolyov
89ba25cd20