Implement complete event editing functionality with backend update endpoint

Frontend Changes:
- Add edit context menu option to EventContextMenu with pencil icon
- Enhance CreateEventModal to support both create and edit modes
- Add event data conversion methods for pre-populating edit forms
- Implement conditional submit logic (on_create vs on_update callbacks)
- Add update_event method to CalendarService with POST /calendar/events/update

Backend Changes:
- Add UpdateEventRequest and UpdateEventResponse models
- Implement update_event handler with event search by UID across calendars
- Add POST /api/calendar/events/update route
- Full validation and parsing of all event properties for updates
- Integrate with existing CalDAV client update_event functionality

Users can now right-click events, select "Edit Event", modify properties in the modal, and successfully update existing events instead of creating duplicates.

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

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

backend/src/lib.rs

@@ -42,6 +42,7 @@ pub async fn run_server() -> Result<(), Box<dyn std::error::Error>> {
         .route("/api/calendar/delete", post(handlers::delete_calendar))
         .route("/api/calendar/events", get(handlers::get_calendar_events))
         .route("/api/calendar/events/create", post(handlers::create_event))
+        .route("/api/calendar/events/update", post(handlers::update_event))
         .route("/api/calendar/events/delete", post(handlers::delete_event))
         .route("/api/calendar/events/:uid", get(handlers::refresh_event))
         .layer(