The base Input component lacked IME composition detection, causing
Enter key presses during CJK input method candidate selection to
mistakenly trigger form submissions.
Add an `onPressEnter` prop with built-in IME safety using
compositionStart/End tracking and nativeEvent.isComposing checks
(with Safari 50ms delay workaround). Migrate all 5 call sites
from manual onKeyDown Enter detection to onPressEnter.
yyh
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