Implement real-time event refresh functionality

- Backend: Add GET /api/calendar/events/:uid endpoint for single event refresh
- Backend: Implement fetch_event_by_uid method to retrieve updated events from CalDAV
- Frontend: Add event click callback system to trigger refresh on interaction
- Frontend: Display loading state with orange pulsing animation during refresh
- Frontend: Smart event data updates without full calendar reload
- Frontend: Graceful error handling with fallback to cached data
- CSS: Add refreshing animation for visual feedback during updates

Events now automatically refresh from CalDAV server when clicked, ensuring
users always see the most current event data including changes made in
other calendar applications.

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

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

backend/src/handlers.rs

@@ -1,5 +1,5 @@
 use axum::{
-    extract::{State, Query},
+    extract::{State, Query, Path},
     http::HeaderMap,
     response::Json,
 };
@@ -73,6 +73,52 @@ pub async fn get_calendar_events(
     Ok(Json(filtered_events))
 }
 
+pub async fn refresh_event(
+    State(_state): State<Arc<AppState>>,
+    Path(uid): Path<String>,
+    headers: HeaderMap,
+) -> Result<Json<Option<CalendarEvent>>, ApiError> {
+    // Verify authentication (extract token from Authorization header)
+    let _token = if let Some(auth_header) = headers.get("authorization") {
+        let auth_str = auth_header
+            .to_str()
+            .map_err(|_| ApiError::Unauthorized("Invalid authorization header".to_string()))?;
+        
+        if auth_str.starts_with("Bearer ") {
+            auth_str.strip_prefix("Bearer ").unwrap().to_string()
+        } else {
+            return Err(ApiError::Unauthorized("Invalid authorization format".to_string()));
+        }
+    } else {
+        return Err(ApiError::Unauthorized("Missing authorization header".to_string()));
+    };
+
+    // TODO: Validate JWT token here
+    
+    // Load CalDAV configuration
+    let config = CalDAVConfig::from_env()
+        .map_err(|e| ApiError::Internal(format!("Failed to load CalDAV config: {}", e)))?;
+    
+    let client = CalDAVClient::new(config);
+    
+    // Discover calendars if needed
+    let calendar_paths = client.discover_calendars()
+        .await
+        .map_err(|e| ApiError::Internal(format!("Failed to discover calendars: {}", e)))?;
+    
+    if calendar_paths.is_empty() {
+        return Ok(Json(None)); // No calendars found
+    }
+    
+    // Fetch the specific event by UID from the first calendar
+    let calendar_path = &calendar_paths[0];
+    let event = client.fetch_event_by_uid(calendar_path, &uid)
+        .await
+        .map_err(|e| ApiError::Internal(format!("Failed to fetch event: {}", e)))?;
+    
+    Ok(Json(event))
+}
+
 pub async fn register(
     State(state): State<Arc<AppState>>,
     Json(request): Json<RegisterRequest>,