Stop let the host go but left the share among those kept for the next launch,
so the next frame took it up again with the same code and secret, and its
guests came straight back. Stopping now forgets it there and on disk, and
Host::stop no longer needs the last handle to the host, so every guest hears
the share stopped even while the dialog still shows it. Sharing again makes a
new code and secret.

Assisted-by: claude-opus-5.5