Implement calendar deletion with right-click context menu

Added complete calendar deletion functionality including:
- Context menu component with right-click activation on calendar items
- Backend API endpoint for calendar deletion with CalDAV DELETE method
- Frontend integration with calendar list refresh after deletion
- Fixed URL construction to prevent double /dav.php path issue
- Added proper error handling and user feedback

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

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

backend/src/lib.rs

@@ -39,6 +39,7 @@ pub async fn run_server() -> Result<(), Box<dyn std::error::Error>> {
         .route("/api/auth/verify", get(handlers::verify_token))
         .route("/api/user/info", get(handlers::get_user_info))
         .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/:uid", get(handlers::refresh_event))
         .layer(