fix(chat-ui): prevent Enter send during IME composition

[pandego]

Summary

Fix chat input Enter handling for CJK IME composition in the dashboard UI.

When users are composing text with an IME, pressing Enter should confirm the current candidate, not send the message.

What changed

• Updated the chat textarea Enter handler in crates/openfang-api/static/index_body.html
• Added !$event.isComposing guard before calling sendMessage()

Issue

Closes #429

Validation

• Manual logic validation against browser IME behavior:
  • Enter sends only when isComposing is false
  • Shift+Enter behavior remains unchanged
• Local Rust checks were not runnable in this environment because cargo is not installed.