fix(circle): do not add hubs/non-circle sites as partner
A hub does not federate (its /.klonkt/actor.json returns 404), but it was
still added to the circle with a dead "error / HTTP 404" row. The add now
rolls back the insert on a 404 and shows a clear message ("a hub cannot be
a circle partner"). Temporary (non-404) errors still leave the link so it
can be refreshed later.
Co-Authored-By: Claude <noreply@…>