Implement complete calendar creation functionality

Add full end-to-end calendar creation feature including:
- Create Calendar button in sidebar footer
- Modal form with name, description, and color picker (16 predefined colors in 4x4 grid)
- Form validation and error handling with loading states
- Backend API endpoint for calendar creation with authentication
- CalDAV MKCALENDAR protocol implementation with proper XML generation
- Real-time calendar list refresh after successful creation
- Responsive design for mobile and desktop

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

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

src/components/mod.rs

@@ -1,7 +1,9 @@
 pub mod login;
 pub mod calendar;
 pub mod event_modal;
+pub mod create_calendar_modal;
 
 pub use login::Login;
 pub use calendar::Calendar;
-pub use event_modal::EventModal;
+pub use event_modal::EventModal;
+pub use create_calendar_modal::CreateCalendarModal;