Use functional components with hooks

Always use functional components with hooks, not class components:
- Simpler, less boilerplate
- Better TypeScript support
- Easier to test
- Hooks enable code reuse without HOCs

```tsx
// Good
function UserProfile({ userId }: { userId: string }) {
const [user, setUser] = useState(null);

useEffect(() => {
fetchUser(userId).then(setUser);
}, [userId]);

if (!user) return ;
return ;
}

// Avoid class components
class UserProfile extends React.Component { ... }
```