fix: prevent Chinese IME Enter from triggering message send

[FarewellSagittarius]

Summary

Fixes #225

When using macOS native Chinese IME and pressing Enter to confirm English text, the message is sent unexpectedly. This happens because compositionend fires before keydown(Enter) in the same event loop tick, so isComposing is already false when Enter is handled.

Fix: Defer the isComposing reset with setTimeout(0) so the Enter keydown in the same cycle is still suppressed.

- const handleCompositionEnd = useCallback(() => setIsComposing(false), []);
+ const handleCompositionEnd = useCallback(() => {
+   setTimeout(() => setIsComposing(false), 0);
+ }, []);

This is a standard pattern used by VS Code, Slack, etc. No impact on non-IME users.

Test plan

• macOS native Chinese IME (Pinyin) → type English → Enter confirms without sending
• Normal Enter (no IME) still sends message
• Shift+Enter still inserts newline
• TypeCheck + 238 unit tests pass

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