fix(desktop): commit IME-finalised text on compositionend so Enter submits

[maxmilian]

Summary

Fixes #39025 (dup #39112 / #39107). On the Windows desktop app, after typing with a Chinese/Japanese/Korean IME, pressing Enter can fail to send: the text is visible in the composer but Hermes behaves as if there is no draft.

Root cause

apps/desktop/src/app/chat/composer/index.tsx skips state writes while IME composition is active:

if (composingRef.current) { return }   // handleEditorInput

and relies on a clean input event after compositionend to push the finalised text into draftRef + aui.composer().setText(...). On Windows/Electron that trailing input event isn't always delivered, leaving:

• visible editor text: non-empty
• tracked draft (draft / hasComposerPayload): empty/stale

Enter then runs submitDraft(), which checks the stale draft and no-ops. (Thanks to @sunwz1115 for the precise diagnosis and a tested fix shape in the issue.)

Fix

Add syncComposerDraft(editor, prevDraft, setText) (in rich-editor.ts) that commits the live contentEditable text back into app state, and call it from compositionend — not just the flaky input event — so the draft is correct the moment composition ends. handleEditorInput reuses the same helper.

Because compositionend is a reliably-delivered event that fires before the user's submit-Enter (a separate event, with a React flush in between), syncing there makes the draft fresh for every downstream path (submitDraft, queueCurrentDraft, hasComposerPayload) without touching any of them — minimal blast radius.

The existing composingRef/nativeEvent.isComposing keydown guard is unchanged, so Enter during active composition still confirms the IME candidate instead of submitting.

Tests

apps/desktop vitest:

• rich-editor.test.ts: syncComposerDraft commits a stale draft, no-ops when already in sync, and is null-editor safe.
• ime-submit-repro.test.tsx: DOM repro — IME-finalised text submits on Enter even when the post-composition input event is dropped; Enter during active composition does not submit. (Mirror-harness style, same as the existing slash-nav-dom-repro.test.tsx, exercising the real syncComposerDraft.)
• tsc -b + eslint clean; all 4 composer test files pass (15 tests).

(Note: the desktop vitest suite has ~52 pre-existing failures in this local environment from a jsdom/Node localStorage quirk, unrelated to this change.)

Scope

• Desktop composer only. Does not touch submitDraft/queueCurrentDraft — syncing on compositionend makes their existing reads correct.
• The separate Ink-TUI premature-submit-during-composition issue ([Bug]: Hermes Desktop TUI 中文输入法 IME 状态下按 Enter/Return 直接提交，而非确认候选词 #39195) is a different component (ui-tui/.../textInput.tsx, no DOM composition events) and is out of scope here.

Follow-up

• If a dropped compositionend is ever observed (not reported), a belt-and-suspenders sync at the top of submitDraft reading live DOM text would cover it.

[maxmilian]

Closing — superseded by the flushEditorToDraft fix (#39614), now on main. The onCompositionEnd handler already flushes finalised IME text into composer state, which is exactly what this PR did via syncDraftFromEditor. No longer needed.