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/services/preferences.rs

@@ -12,6 +12,7 @@ pub struct UserPreferences {
     pub calendar_view_mode: Option<String>,
     pub calendar_theme: Option<String>,
     pub calendar_colors: Option<String>,
+    pub last_used_calendar: Option<String>,
 }
 
 #[derive(Debug, Serialize)]
@@ -22,6 +23,7 @@ pub struct UpdatePreferencesRequest {
     pub calendar_view_mode: Option<String>,
     pub calendar_theme: Option<String>,
     pub calendar_colors: Option<String>,
+    pub last_used_calendar: Option<String>,
 }
 
 #[allow(dead_code)]
@@ -61,6 +63,7 @@ impl PreferencesService {
             calendar_view_mode: None,
             calendar_theme: None,
             calendar_colors: None,
+            last_used_calendar: None,
         });
         
         // Update the specific field
@@ -95,6 +98,7 @@ impl PreferencesService {
             calendar_view_mode: preferences.calendar_view_mode.clone(),
             calendar_theme: preferences.calendar_theme.clone(),
             calendar_colors: preferences.calendar_colors.clone(),
+            last_used_calendar: preferences.last_used_calendar.clone(),
         };
         
         self.sync_preferences(&session_token, &request).await
@@ -156,6 +160,7 @@ impl PreferencesService {
             calendar_view_mode: LocalStorage::get::<String>("calendar_view_mode").ok(),
             calendar_theme: LocalStorage::get::<String>("calendar_theme").ok(),
             calendar_colors: LocalStorage::get::<String>("calendar_colors").ok(),
+            last_used_calendar: LocalStorage::get::<String>("last_used_calendar").ok(),
         };
         
         // Only migrate if we have some preferences to migrate
@@ -164,6 +169,7 @@ impl PreferencesService {
             || request.calendar_view_mode.is_some()
             || request.calendar_theme.is_some()
             || request.calendar_colors.is_some()
+            || request.last_used_calendar.is_some()
         {
             self.sync_preferences(&session_token, &request).await?;
             
@@ -177,4 +183,24 @@ impl PreferencesService {
         
         Ok(())
     }
+
+    /// Update the last used calendar and sync with backend
+    pub async fn update_last_used_calendar(&self, calendar_path: &str) -> Result<(), String> {
+        // Get session token
+        let session_token = LocalStorage::get::<String>("session_token")
+            .map_err(|_| "No session token found".to_string())?;
+        
+        // Create minimal update request with only the last used calendar
+        let request = UpdatePreferencesRequest {
+            calendar_selected_date: None,
+            calendar_time_increment: None,
+            calendar_view_mode: None,
+            calendar_theme: None,
+            calendar_colors: None,
+            last_used_calendar: Some(calendar_path.to_string()),
+        };
+        
+        // Sync to backend
+        self.sync_preferences(&session_token, &request).await
+    }
 }