Missing useUser hook in VNDA integration

Similar to the "useUser" that VTEX has

```
import { state as storeState } from "./context.ts";

const { user, loading } = storeState;

const state = { user, loading };

export const useUser = () => state;
```

Locate the VNDA integration and its context.ts, then compare them with the existing VTEX useUser implementation shown in the issue. Done means the VNDA integration exposes an equivalent useUser hook with user and loading state; the issue names no tests or other verification steps.

Written by the indexing model from the issue text.