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/db.rs

@@ -95,6 +95,7 @@ pub struct UserPreferences {
     pub calendar_theme: Option<String>,
     pub calendar_style: Option<String>,
     pub calendar_colors: Option<String>, // JSON string
+    pub last_used_calendar: Option<String>,
     pub updated_at: DateTime<Utc>,
 }
 
@@ -109,6 +110,7 @@ impl UserPreferences {
             calendar_theme: Some("light".to_string()),
             calendar_style: Some("default".to_string()),
             calendar_colors: None,
+            last_used_calendar: None,
             updated_at: Utc::now(),
         }
     }
@@ -266,8 +268,8 @@ impl<'a> PreferencesRepository<'a> {
             sqlx::query(
                 "INSERT INTO user_preferences 
                  (user_id, calendar_selected_date, calendar_time_increment, 
-                  calendar_view_mode, calendar_theme, calendar_style, calendar_colors, updated_at) 
-                 VALUES (?, ?, ?, ?, ?, ?, ?, ?)",
+                  calendar_view_mode, calendar_theme, calendar_style, calendar_colors, last_used_calendar, updated_at) 
+                 VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)",
             )
             .bind(&prefs.user_id)
             .bind(&prefs.calendar_selected_date)
@@ -276,6 +278,7 @@ impl<'a> PreferencesRepository<'a> {
             .bind(&prefs.calendar_theme)
             .bind(&prefs.calendar_style)
             .bind(&prefs.calendar_colors)
+            .bind(&prefs.last_used_calendar)
             .bind(&prefs.updated_at)
             .execute(self.db.pool())
             .await?;
@@ -290,7 +293,7 @@ impl<'a> PreferencesRepository<'a> {
             "UPDATE user_preferences 
              SET calendar_selected_date = ?, calendar_time_increment = ?, 
                  calendar_view_mode = ?, calendar_theme = ?, calendar_style = ?,
-                 calendar_colors = ?, updated_at = ?
+                 calendar_colors = ?, last_used_calendar = ?, updated_at = ?
              WHERE user_id = ?",
         )
         .bind(&prefs.calendar_selected_date)
@@ -299,6 +302,7 @@ impl<'a> PreferencesRepository<'a> {
         .bind(&prefs.calendar_theme)
         .bind(&prefs.calendar_style)
         .bind(&prefs.calendar_colors)
+        .bind(&prefs.last_used_calendar)
         .bind(Utc::now())
         .bind(&prefs.user_id)
         .execute(self.db.pool())