Implement complete event creation functionality with CalDAV backend

- Add CalDAV create_event method with proper iCalendar generation
- Add comprehensive backend API for event creation with validation
- Add event creation models and handlers with date/time parsing
- Add frontend service method for creating events via API
- Update frontend to call backend API instead of placeholder
- Fix CalDAV URL construction to avoid duplicate /dav.php paths
- Support all event fields: title, description, dates, location, all-day

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

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

backend/src/lib.rs

@@ -41,6 +41,7 @@ pub async fn run_server() -> Result<(), Box<dyn std::error::Error>> {
         .route("/api/calendar/create", post(handlers::create_calendar))
         .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/delete", post(handlers::delete_event))
         .route("/api/calendar/events/:uid", get(handlers::refresh_event))
         .layer(