Implement last used calendar tracking with localStorage and database sync

- Add database migration for last_used_calendar field in user preferences
- Update backend models and handlers to support last_used_calendar persistence
- Modify frontend preferences service with update_last_used_calendar() method
- Implement automatic saving of selected calendar on event creation
- Add localStorage fallback for offline usage and immediate UI response
- Update create event modal to default to last used calendar for new events
- Clean up unused imports from event form components

🤖 Generated with [Claude Code](https://claude.ai/code)

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

backend/src/auth.rs

@@ -93,6 +93,7 @@ impl AuthService {
                         calendar_theme: preferences.calendar_theme,
                         calendar_style: preferences.calendar_style,
                         calendar_colors: preferences.calendar_colors,
+                        last_used_calendar: preferences.last_used_calendar,
                     },
                 })
             }