Both onKeyDown handlers in Composer.tsx committed on Enter without checking
IME composition state, so confirming a Korean/Japanese/Chinese composition
with Enter re-committed the composing character (한글 -> 한글글).
Extract a shouldSubmitOnEnter predicate that returns false while
nativeEvent.isComposing or a composing ref is set, and a useCompositionGuard
hook that tracks compositionstart/compositionend with a deferred reset for
the WebKit case where the confirming Enter keydown fires after
compositionend. Wire both AskEditor and Composer to it.
Fixes#6163
roboomp
27df758721