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/services/calendar_service.rs

@@ -467,6 +467,67 @@ impl CalendarService {
         (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)
     }
 
+    /// Create a new calendar on the CalDAV server
+    pub async fn create_calendar(
+        &self, 
+        token: &str, 
+        password: &str,
+        name: String,
+        description: Option<String>,
+        color: Option<String>
+    ) -> Result<(), String> {
+        let window = web_sys::window().ok_or("No global window exists")?;
+        
+        let opts = RequestInit::new();
+        opts.set_method("POST");
+        opts.set_mode(RequestMode::Cors);
+
+        let body = serde_json::json!({
+            "name": name,
+            "description": description,
+            "color": color
+        });
+
+        let body_string = serde_json::to_string(&body)
+            .map_err(|e| format!("JSON serialization failed: {}", e))?;
+
+        opts.set_body(&body_string.into());
+
+        let url = format!("{}/calendar/create", self.base_url);
+        let request = Request::new_with_str_and_init(&url, &opts)
+            .map_err(|e| format!("Request creation failed: {:?}", e))?;
+
+        request.headers().set("Authorization", &format!("Bearer {}", token))
+            .map_err(|e| format!("Authorization header setting failed: {:?}", e))?;
+        
+        request.headers().set("X-CalDAV-Password", password)
+            .map_err(|e| format!("Password header setting failed: {:?}", e))?;
+
+        request.headers().set("Content-Type", "application/json")
+            .map_err(|e| format!("Content-Type header setting failed: {:?}", e))?;
+
+        let resp_value = JsFuture::from(window.fetch_with_request(&request))
+            .await
+            .map_err(|e| format!("Network request failed: {:?}", e))?;
+
+        let resp: Response = resp_value.dyn_into()
+            .map_err(|e| format!("Response cast failed: {:?}", e))?;
+
+        let text = JsFuture::from(resp.text()
+            .map_err(|e| format!("Text extraction failed: {:?}", e))?)
+            .await
+            .map_err(|e| format!("Text promise failed: {:?}", e))?;
+
+        let text_string = text.as_string()
+            .ok_or("Response text is not a string")?;
+
+        if resp.ok() {
+            Ok(())
+        } else {
+            Err(format!("Request failed with status {}: {}", resp.status(), text_string))
+        }
+    }
+
     /// Refresh a single event by UID from the CalDAV server
     pub async fn refresh_event(&self, token: &str, password: &str, uid: &str) -> Result<Option<CalendarEvent>, String> {
         let window = web_sys::window().ok_or("No global window exists")?;