Use React Query or SWR for data fetching

Use a data fetching library instead of useEffect + fetch:
- Automatic caching and revalidation
- Loading and error states
- Optimistic updates
- Request deduplication

```tsx
import { useQuery } from '@tanstack/react-query';

function UserList() {
const { data: users, isLoading, error } = useQuery({
queryKey: ['users'],
queryFn: () => fetch('/api/users').then(r => r.json()),
});

if (isLoading) return ;
if (error) return ;
return ;
}
```