Fix desktop IME composition submit handling

[wbcoder0]

What changed

• Add a shared desktop composer IME detection helper.
• Prevent Enter from submitting while the main composer or edit composer is still composing IME text.
• Sync finalized contentEditable text on compositionend.
• Add focused tests for IME composition signals, including the process-key 229 fallback.

Why

Chinese/Japanese/Korean IMEs use Enter to confirm candidate text. The desktop composer could treat that Enter as a submit action, sending the prompt before the user had finalized the intended text.

Validation

• npm -w apps/desktop run test:ui -- src/app/chat/composer/ime.test.ts src/app/chat/composer/slash-nav-dom-repro.test.tsx
• npm -w apps/desktop run type-check

[alt-glitch]

Duplicate of the IME-composition-Enter fix cluster for #37483. There are several competing open PRs implementing the same ime.ts (isComposing + legacy keyCode 229 + Process-key 229) guard in the desktop composer: #37996, #37757, #38785 (and earlier #37487/#38135/#38247). Maintainers should pick one canonical PR and close the rest.

[teknium1]

Thanks for covering both composition signals and the edit-composer path.

Problems

• The PR currently conflicts because its edit-composer change targets the former apps/desktop/src/components/assistant-ui/thread.tsx; current main moved that component to apps/desktop/src/components/assistant-ui/thread/user-edit-composer.tsx (commits 7ff6908a5 and fa7bce078).
• The remaining live defect is in apps/desktop/src/components/assistant-ui/thread/user-edit-composer.tsx:527-577: unmodified Enter calls submitEdit() without a composition guard, and the editor at :627-652 has no composition handlers.
• ime.test.ts checks the helper only; it does not cover that edit-composer submit path.

Suggested changes

• Port the edit-composer guard and composition-end synchronization to thread/user-edit-composer.tsx, and add a DOM regression test proving an IME Enter does not submit or select a trigger item.
• Preserve current main's existing main-composer behavior at apps/desktop/src/app/chat/composer/index.tsx:364-372 and :727-741; apply the 229 fallback to that current implementation rather than transplanting the stale snapshot.

Automated hermes-sweeper review.