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/models.rs

@@ -34,6 +34,19 @@ pub struct CalendarInfo {
     pub color: String,
 }
 
+#[derive(Debug, Deserialize)]
+pub struct CreateCalendarRequest {
+    pub name: String,
+    pub description: Option<String>,
+    pub color: Option<String>,
+}
+
+#[derive(Debug, Serialize)]
+pub struct CreateCalendarResponse {
+    pub success: bool,
+    pub message: String,
+}
+
 // Error handling
 #[derive(Debug)]
 pub enum ApiError {