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/calendar.rs

@@ -590,6 +590,74 @@ impl CalDAVClient {
         
         Err(CalDAVError::ParseError(format!("Unable to parse datetime: {}", datetime_str)))
     }
+
+    /// Create a new calendar on the CalDAV server using MKCALENDAR
+    pub async fn create_calendar(&self, name: &str, description: Option<&str>, color: Option<&str>) -> Result<(), CalDAVError> {
+        // Sanitize calendar name for URL path
+        let calendar_id = name
+            .chars()
+            .map(|c| if c.is_alphanumeric() { c } else { '-' })
+            .collect::<String>()
+            .to_lowercase();
+        
+        let calendar_path = format!("/calendars/{}/{}/", self.config.username, calendar_id);
+        let full_url = format!("{}{}", self.config.server_url.trim_end_matches('/'), calendar_path);
+        
+        // Build color property if provided
+        let color_property = if let Some(color) = color {
+            format!(r#"<ic:calendar-color xmlns:ic="http://apple.com/ns/ical/">{}</ic:calendar-color>"#, color)
+        } else {
+            String::new()
+        };
+        
+        let description_property = if let Some(desc) = description {
+            format!(r#"<c:calendar-description xmlns:c="urn:ietf:params:xml:ns:caldav">{}</c:calendar-description>"#, desc)
+        } else {
+            String::new()
+        };
+        
+        // Create the MKCALENDAR request body
+        let mkcalendar_body = format!(
+            r#"<?xml version="1.0" encoding="utf-8" ?>
+<c:mkcalendar xmlns:d="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav" xmlns:ic="http://apple.com/ns/ical/">
+    <d:set>
+        <d:prop>
+            <d:displayname>{}</d:displayname>
+            <c:supported-calendar-component-set>
+                <c:comp name="VEVENT"/>
+            </c:supported-calendar-component-set>
+            {}
+            {}
+        </d:prop>
+    </d:set>
+</c:mkcalendar>"#,
+            name, color_property, description_property
+        );
+        
+        println!("Creating calendar at: {}", full_url);
+        println!("MKCALENDAR body: {}", mkcalendar_body);
+        
+        let response = self.http_client
+            .request(reqwest::Method::from_bytes(b"MKCALENDAR").unwrap(), &full_url)
+            .header("Content-Type", "application/xml; charset=utf-8")
+            .header("Authorization", format!("Basic {}", self.config.get_basic_auth()))
+            .body(mkcalendar_body)
+            .send()
+            .await
+            .map_err(|e| CalDAVError::ParseError(e.to_string()))?;
+
+        println!("Calendar creation response status: {}", response.status());
+        
+        if response.status().is_success() {
+            println!("✅ Calendar created successfully at {}", calendar_path);
+            Ok(())
+        } else {
+            let status = response.status();
+            let error_body = response.text().await.unwrap_or_default();
+            println!("❌ Calendar creation failed: {} - {}", status, error_body);
+            Err(CalDAVError::ServerError(status.as_u16()))
+        }
+    }
 }
 
 /// Helper struct for extracting calendar data from XML responses
@@ -688,7 +756,9 @@ mod tests {
         }
     }
 
-    /// Test parsing a sample iCal event
+}
+
+/// Test parsing a sample iCal event
     #[test]
     fn test_parse_ical_event() {
         let sample_ical = r#"BEGIN:VCALENDAR
@@ -780,4 +850,3 @@ END:VCALENDAR"#;
         
         println!("✓ Event enum tests passed!");
     }
-}