fix(web-ui): keep inline edit keys with IME

[QZQ-go]

Summary

• keep IME-owned Enter and Escape events inside the plain inline message editor
• generalize the shared IME guard API so it covers keyboard actions beyond Enter
• add focused coverage for tracked composition, native isComposing, keyCode 229, normal follow-up actions, and the existing rich editor boundary

Fixes #2344

Type and Areas

Type: Regression fix

Areas: Web UI input handling

Motivation / Impact

Confirming or dismissing a Chinese IME candidate no longer submits or cancels an inline edit. Normal Enter submission, Escape cancellation, modifier behavior, mention handling, and button actions remain unchanged.

The change is limited to shared browser input handling. It does not alter persistence, runtime, remote transport, rollback semantics, or protocol compatibility. Remote-workspace edit-and-rerun remains unsupported by current product policy.

Verification

• pnpm --dir src/web-ui exec vitest run src/flow_chat/components/modern/UserMessageEditComposer.test.tsx: 8 tests passed
• pnpm run type-check:web: passed
• pnpm --dir src/web-ui run gen:types: 34 export tests passed; generated 45 ignored TypeScript bindings required by type checking
• macOS desktop smoke test: pending human verification, per the module instruction not to perform UI interaction verification

Reviewer Notes

This PR was developed with AI assistance. Automated focused tests and Web UI type checking passed; manual macOS IME verification is pending.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.