"Open on <machine>" showed for any destination in the catalog; it now
needs that environment to be connected.

The thread header offered "Continue on…" to a connection that cannot
change threads, and both it and the row menu offered it while a turn was
live or queued, which the server refuses. Both now check that first.

A message waiting in this device's outbox would be refused once its
thread starts moving, losing it. The sheet now says so and holds the
move until that message is sent or discarded.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>