Moves the chat-focused Esc-cancel listener (the latest-handler ref + the
register-once window keydown effect) out of ChatBar into
hooks/use-composer-esc-cancel.ts, verbatim. Encapsulating the latest-closure ref
inside its own hook is the first of the plan's "delete the latest-closure refs"
cleanups: it's no longer a loose ref in the 1.4k-line component, just an
implementation detail of a focused side-effect hook keyed on busy/awaitingInput/
onCancel.
e009ba57ac