Complete frontend migration to shared VEvent model

- Update frontend to use shared calendar-models library
- Add display methods to VEvent for UI compatibility
- Remove duplicate VEvent definitions in frontend
- Fix JSON field name mismatch (dtstart/dtend vs start/end)
- Create CalendarEvent type alias for backward compatibility
- Resolve "missing field start" parsing error

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/components/route_handler.rs

@@ -159,7 +159,7 @@ pub fn calendar_view(props: &CalendarViewProps) -> Html {
                     
                     match calendar_service.refresh_event(&token, &password, &uid).await {
                         Ok(Some(refreshed_event)) => {
-                            let refreshed_vevent = VEvent::from_calendar_event(&refreshed_event);
+                            let refreshed_vevent = refreshed_event; // CalendarEvent is now VEvent
                             let mut updated_events = (*events).clone();
                             
                             for (_, day_events) in updated_events.iter_mut() {