fix(desktop): respect IME composition in composer

[momiji-manjyuu]

Summary

• Add an IME composition keyboard-event helper for React/Electron key quirks
• Ignore composer keydown/keyup handling while text is composing so Enter can confirm Japanese/Chinese/Korean conversion
• Add regression coverage for composition flags, keyCode 229, Process key, and normal Enter

Test Plan

• npm run test:ui -- src/app/chat/composer/text-utils.test.ts src/app/chat/composer/slash-nav-dom-repro.test.tsx src/app/chat/composer/rich-editor.test.ts
• npm run type-check
• npm run build

[teknium1]

Thanks for the focused IME regression work. The Desktop composer behavior in this PR is already implemented on current main.

Automated hermes-sweeper review:

• apps/desktop/src/app/chat/composer/index.tsx:364-372 returns before Enter submission and trigger handling while composingRef.current or event.nativeEvent.isComposing is true.
• apps/desktop/src/app/chat/composer/index.tsx:727-740 tracks composition lifecycle and flushes finalized IME text on compositionend, covering Chromium paths that omit a trailing input event.
• Commit 0caa23788f6016d3b3216b22fa0203a93152bb99 (merged PR fix(desktop): prevent IME Enter from splitting messages and viewport resize from disarming scroll anchor #38333) introduced the composing-Enter protection; commit 8e629b9f386d12b726bccb32e9d7b48402ea73ea added committed-text synchronization. Current regression coverage is in apps/desktop/src/app/chat/composer/ime-composition-dom-repro.test.tsx:61-106.

Closing as implemented on main.