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/app.rs

@@ -413,6 +413,21 @@ pub fn App() -> Html {
         let auth_token = auth_token.clone();
         Callback::from(move |event_data: EventCreationData| {
             web_sys::console::log_1(&format!("Creating event: {:?}", event_data).into());
+            
+            // Save the selected calendar as the last used calendar
+            if let Some(ref calendar_path) = event_data.selected_calendar {
+                let _ = LocalStorage::set("last_used_calendar", calendar_path);
+                
+                // Also sync to backend asynchronously
+                let calendar_path_for_sync = calendar_path.clone();
+                wasm_bindgen_futures::spawn_local(async move {
+                    let preferences_service = crate::services::preferences::PreferencesService::new();
+                    if let Err(e) = preferences_service.update_last_used_calendar(&calendar_path_for_sync).await {
+                        web_sys::console::warn_1(&format!("Failed to sync last used calendar to backend: {}", e).into());
+                    }
+                });
+            }
+            
             create_event_modal_open.set(false);
 
             if let Some(_token) = (*auth_token).clone() {