ui: keep queued steering messages in their own conversation
A message queued while a conversation was busy was re-sent through
sendMessage, which targets the active conversation, so switching chats
before the turn ended delivered it into the wrong one. Re-send only when
the owning conversation is active; otherwise keep it queued there and
flush it when that conversation is opened again.
Assisted-by: Claude (Opus 5.5 Medium)
