- Add user_preferences table to store per-user settings (auto_save_drafts, markdown_preview_default)
- Create UserPreferences model with GetUserPreferences, CreateDefaultPreferences, and UpdateUserPreferences functions
- Add PreferencesHandler for GET/POST requests to display and save user preferences
- Create preferences.html template with checkbox for draft auto-save and radio buttons for markdown preview default
- Add "Preferences" link to navbar for logged-in users
- Register /preferences/ route with login requirement
This establishes the foundation for advanced features like draft auto-save and markdown preview toggle, allowing users to customize their experience.