### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

Typing in the Playground chat box with an IME (Chinese Zhuyin/Pinyin, Japanese, Korean) sends the message prematurely.

With these input methods, Enter is used to confirm a candidate from the conversion popup. The Playground treats that Enter as "submit", so a half-composed message gets sent and the rest of what the user was typing is lost. In practice this makes the Playground close to unusable for CJK users — nearly every phrase needs at least one candidate confirmation.

**Expected:** Enter confirms the IME candidate and the message stays in the textarea. Only a non-composing Enter should send.

### Steps to Reproduce

1. Switch the OS to a CJK input method (e.g. macOS 注音 / Zhuyin, or Pinyin) 2. Open the Playground chat at `/ui/playground/` 3. Type a phrase that requires candidate selection, and press Enter to pick a candidate from the popup 4. Observe the partially-composed message being sent instead of the candidate being confirmed

### Root cause

The keydown handlers only check `event.key === "Enter"` and never check whether an IME compo…