fix: `auth.service` reacted inconsistently to `username` changes

fix: `auth.service` reacted inconsistently to `username` changes

* The subscription for `error` is not required, because the subscription for
  `username` is also triggered, when the username is reset after an error.
* The `login` function must not manipulate the local variable `username`
  at all.
* The separate logic in the app-components's init-hook, that redirects
  to the login page, when the user is not yet known, is no longer
  necessary, since the routing can also be triggered in the subscription
  for `username`.