"Open on <machine>" showed for a destination this client knew about but
was not connected to; it now needs a connected environment.

The thread menus waited for move targets and then showed the menu even
if the user had since closed it, opened another, or switched threads.
Each open now takes a token, and a stale one is dropped before showing.

Preview annotations and app messages reached onSend and sendAppMessage
directly, past the closed send button, so they could clear the composer
for a thread that is moving away. Both now check the handoff first.

The handoff notice sat ahead of the system notices, so on a priority tie
it hid an update or reconnect notice in the collapsed stack.

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