fix: guard desktop Enter submit during IME processing

[stomeonst]

Summary

• Add a shared desktop IME key guard that treats Chromium/Electron keyCode 229 as active IME composition.
• Use the guard for the main chat composer and the edit-message composer so Enter confirms CJK preedit text instead of submitting/selecting actions.
• Add regression coverage for keyCode 229 and keep the existing IME composition/Enter race tests passing.

Testing

• npm run test:ui -- --run src/app/chat/composer/ime.test.ts src/app/chat/composer/enter-submit-dom-race.test.tsx src/app/chat/composer/ime-composition-dom-repro.test.tsx
• npm run type-check
• npm run build

[alt-glitch]

Duplicate of #38794 — both add an IME keyCode-229 / composition guard so confirming CJK preedit text via Enter no longer submits the message. This is part of the heavily-competed #37483 fix cluster (siblings #40015, #38785). #38794 is the earliest open canonical fix.

[stomeonst]

Thanks for the pointer. Closing this as a duplicate of #38794. I’ll keep tracking the canonical fix there.