feat: If not known, an existing user can be selected or a new one created

feat: If not known, an existing user can be selected or a new one created

* Introduced the method `setUser(id: string)` in `UserService`.
* The method fetches the user from the backend and remembers it.
* Also updated the pact between the `UserService` and the
  `ChatBackendController`.
* Remodeld the `user.component` accordingly.