Fix duplicate chat submits from Enter races and IME composition

[coconiiruzo]

Summary

• Guard chat submit with a synchronous localTurnRef update so a second Enter/click cannot race ahead of React state and POST a duplicate agent turn.
• Ignore Enter while an IME composition is active (isComposing / keyCode 229), which is a common Japanese-input footgun in chat composers.

Screenshot

One session showing before/after:

• Before: 「こんにちは」 and 「あなたは？」 each produced two user bubbles and two agent turns.
• After this fix: the final 「ハロー」 turn stayed a single request/response.

Test plan

• Fresh chat: type Japanese with IME, press Enter once to confirm composition — message should not send
• Press Enter again after composition ends — exactly one user message and one agent turn
• Rapid double Enter / double click Send on English text — only one turn is created (second becomes pending queue item if first is already busy)
• Stop and retry still work as before

Local verification: bun run check — 692 pass, 0 fail (Bun 1.3.14).