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>

src/components/mod.rs

@@ -2,8 +2,10 @@ pub mod login;
 pub mod calendar;
 pub mod event_modal;
 pub mod create_calendar_modal;
+pub mod context_menu;
 
 pub use login::Login;
 pub use calendar::Calendar;
 pub use event_modal::EventModal;
-pub use create_calendar_modal::CreateCalendarModal;
+pub use create_calendar_modal::CreateCalendarModal;
+pub use context_menu::ContextMenu;