Prevent Enter from sending during IME composition

[57wuchi]

Summary

• prevent the composer from sending when Enter is used to confirm an active IME composition
• preserve the existing Enter-to-send and Shift+Enter-to-newline behavior after composition ends
• document the keyboard behavior in both English and Traditional Chinese

Problem

When typing Chinese or other text with an input method editor, Enter is used to confirm the underlined composition candidate. The composer currently handles that same keydown as a send action, which can submit incomplete text before the user has finished composing.

Implementation

• track the textarea's compositionstart and compositionend lifecycle
• return from handleKeydown while either the local composition flag or KeyboardEvent.isComposing is active
• run the composition guard before both @mention selection and message-send handling
• add a regression contract test for the composition lifecycle and guard ordering
• update README.md and README.zh-TW.md with the expected keyboard behavior

Expected behavior

1. While IME composition is underlined, Enter confirms the selected text without sending.
2. After composition ends, the next Enter sends the message.
3. Shift+Enter continues to insert a new line.

Testing

• node scripts/build.js
• node --test test/*.test.js
• 31 tests passed