No Theme Detection Hook

Currently no way to respond to theme changes:
```
// Current: Manual check
const theme = window.openai?.theme;

// Should have:
import { useTheme } from '@ainativekit/ui';

function MyComponent() {
  const theme = useTheme(); // 'light' | 'dark', updates on change
  
  return <div>Current theme: {theme}</div>;
}
```