fix(desktop): guard IME composition Enter key on macOS Chromium edge case

[aotian16]

Problem

When using a Chinese IME on macOS (e.g. macOS Pinyin), pressing Enter to confirm an IME candidate sometimes submits the draft message instead of committing the composed text.

Root Cause

The existing IME guards check composingRef.current (set by compositionstart/compositionend) and event.nativeEvent.isComposing. However, macOS Chinese IMEs trigger a known Chromium edge case where the Enter keydown event is dispatched after compositionend without isComposing=true. Both guards miss this window, so Enter reaches submitDraft().

Fix

Introduce recentCompositionRef, set to true on compositionend and cleared after a 100ms timeout. The keydown handler now checks composingRef.current || event.nativeEvent.isComposing || recentCompositionRef.current, which covers all IME timing scenarios.

Testing

1. Open Hermes Desktop
2. Switch to Chinese Pinyin IME
3. Type English (or Pinyin) characters
4. Press Enter to confirm the IME candidate
5. Press Enter again (empty draft to submit) — should submit, not compose

[alt-glitch]

This was generated by AI during triage.

Related: #37483 (canonical issue — Enter sends message during IME composition). Competing/alternate fix to #37487 (adds !isComposing guard to the Enter handlers). This PR instead adds a recentCompositionRef 100ms window to catch the macOS Chromium case where the Enter keydown arrives after compositionend without isComposing=true. Maintainer to pick between the two mechanisms.

[teknium1]

Thanks for targeting the post-compositionend Chromium sequence; current main still lacks that guard at apps/desktop/src/app/chat/composer/index.tsx:370.

Problems

• apps/desktop/src/app/chat/composer/index.tsx:736 schedules an independent clear on every composition end. If two compositions finish within 100 ms, the first callback can clear recentCompositionRef before the second window expires, re-opening the reported submit path.
• Commit c4751749d0ba13cb3deaa1c94404f2900899cb7f adds no regression test. The existing apps/desktop/src/app/chat/composer/ime-composition-dom-repro.test.tsx:61 covers draft synchronization, not an Enter dispatched after compositionend with isComposing=false.

Suggested changes

• Cancel or supersede the prior timer so only the latest composition-end window can clear the guard, and clean it up on unmount.
• Add a fake-timer regression test for the stated event ordering, including consecutive compositions and a normal Enter after expiration.

Automated hermes-sweeper review.

[teknium1]

Each composition end creates an independent timer. If another composition ends before this callback runs, this older callback can clear the shared ref during the newer composition's 100 ms window. Store and cancel the prior timer (or use a generation token) before scheduling this callback.