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>

frontend/src/components/mod.rs

@@ -24,10 +24,6 @@ pub use create_calendar_modal::CreateCalendarModal;
 pub use create_event_modal::CreateEventModal;
 // Re-export event form types for backwards compatibility
 pub use event_form::EventCreationData;
-pub use event_form::{
-    EventClass as EventFormClass, EventCreationData as EventFormData, EventStatus as EventFormStatus, 
-    RecurrenceType as EventFormRecurrenceType, ReminderType as EventFormReminderType,
-};
 pub use event_context_menu::{DeleteAction, EditAction, EventContextMenu};
 pub use event_modal::EventModal;
 pub use login::Login;