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>

backend/src/lib.rs

@@ -38,6 +38,7 @@ pub async fn run_server() -> Result<(), Box<dyn std::error::Error>> {
         .route("/api/auth/login", post(handlers::login))
         .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/events", get(handlers::get_calendar_events))
         .route("/api/calendar/events/:uid", get(handlers::refresh_event))
         .layer(