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/handlers/preferences.rs

@@ -40,6 +40,7 @@ pub async fn get_preferences(
         calendar_theme: preferences.calendar_theme,
         calendar_style: preferences.calendar_style,
         calendar_colors: preferences.calendar_colors,
+        last_used_calendar: preferences.last_used_calendar,
     }))
 }
 
@@ -85,6 +86,9 @@ pub async fn update_preferences(
     if request.calendar_colors.is_some() {
         preferences.calendar_colors = request.calendar_colors;
     }
+    if request.last_used_calendar.is_some() {
+        preferences.last_used_calendar = request.last_used_calendar;
+    }
 
     prefs_repo
         .update(&preferences)
@@ -100,6 +104,7 @@ pub async fn update_preferences(
             calendar_theme: preferences.calendar_theme,
             calendar_style: preferences.calendar_style,
             calendar_colors: preferences.calendar_colors,
+            last_used_calendar: preferences.last_used_calendar,
         }),
     ))
 }