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/create_event_modal.rs

@@ -2,6 +2,7 @@ use crate::components::event_form::*;
 use crate::components::EditAction;
 use crate::models::ical::VEvent;
 use crate::services::calendar_service::CalendarInfo;
+use gloo_storage::{LocalStorage, Storage};
 use yew::prelude::*;
 
 #[derive(Properties, PartialEq)]
@@ -57,7 +58,25 @@ pub fn create_event_modal(props: &CreateEventModalProps) -> Html {
 
                 // Set default calendar
                 if data.selected_calendar.is_none() && !available_calendars.is_empty() {
-                    data.selected_calendar = Some(available_calendars[0].path.clone());
+                    // For new events, try to use the last used calendar
+                    if event_to_edit.is_none() {
+                        // Try to get last used calendar from localStorage
+                        if let Ok(last_used_calendar) = LocalStorage::get::<String>("last_used_calendar") {
+                            // Check if the last used calendar is still available
+                            if available_calendars.iter().any(|cal| cal.path == last_used_calendar) {
+                                data.selected_calendar = Some(last_used_calendar);
+                            } else {
+                                // Fall back to first available calendar
+                                data.selected_calendar = Some(available_calendars[0].path.clone());
+                            }
+                        } else {
+                            // No last used calendar, use first available
+                            data.selected_calendar = Some(available_calendars[0].path.clone());
+                        }
+                    } else {
+                        // For editing existing events, keep the current calendar as default
+                        data.selected_calendar = Some(available_calendars[0].path.clone());
+                    }
                 }
 
                 // Set edit scope if provided