fix(chat): ignore composer Enter during IME composition

[boss477]

Summary

• avoid submitting the chat composer when Enter is pressed during active IME composition
• keep normal Enter-to-send and Shift+Enter multiline behavior unchanged
• add focused regression coverage for composing, legacy keyCode 229, normal Enter, and Shift+Enter

Fixes #37

Tests

• npm test -- --run tests/chat-composer.test.ts tests/transcript.test.tsx
• npm run typecheck
• npm run lint
• npx prettier --check src/client/Chat.tsx src/client/chat-composer.ts tests/chat-composer.test.ts

[jerelvelarde]

Prevents Enter from submitting incomplete IME input while preserving normal Enter and Shift+Enter behavior. Strong accessibility and international input fit for the template. No new dependencies, authority, or security blockers found. Regression coverage includes active composition and legacy keyCode 229.

Validation on the reviewed head: 168 tests passed; typecheck, lint, formatting, and production build passed.