fix: reduce IME composition end Enter tail window from 80ms to 20ms

[Is14w]

Linked issue

Closes #21

Summary

The 80ms unconditional suppression window after compositionend was too aggressive — it blocked Enter when users selected IME candidates with non-Enter keys (e.g. number keys) and immediately pressed Enter to send.

20ms is tight enough to still catch the inseparable compositionend→Enter keydown pairs in problematic IME implementations, while avoiding false positives from fast typists.

Change scope

• Modules: agent-gui (desktop frontend), agent-gateway (WebUI mirror)
• Key paths:
  • crates/agent-gui/src/components/chat/MentionComposer.tsx
  • crates/agent-gateway/web/src/components/chat/MentionComposer.tsx

Screenshots / preview

N/A — this is a single-constant timing change (80ms → 20ms) with no visual effect. The behavior difference is only perceptible by fast typists using IME number-key selection followed immediately by Enter.

Verification

• git diff --check — clean, no trailing whitespace
• node scripts/check-mirror.mjs — 115 files pass, GUI/WebUI mirrors byte-identical
• cargo check --manifest-path crates/agent-gui/src-tauri/Cargo.toml --tests — pass (no new warnings)
• pnpm test:frontend — 2 pre-existing failures unrelated to this change (mention-refetch, presets sync)
• pnpm lint — pre-existing errors, none introduced by this change
• Gateway Go tests — skipped, no Go code changed

Pre-submit checklist

• A requirement issue is linked (or this is a trivial fix that needs no issue, as explained in the summary).
• Synced with the target branch; no merge conflicts.
• The change is focused, with no unrelated modifications.
• No secrets, tokens, or personal data included.
• Docs are updated for changes affecting user behavior, deployment, or configuration.

[github-actions]

PR governance checks passed. Awaiting human review.