feat(i18n): add Vietnamese translations for docs and WebUI

[thanhnn91qn-afk]

Summary / Tóm tắt

Add Vietnamese (vi) language support across user-facing surfaces so DS2API is friendlier to Vietnamese developers.

Bổ sung hỗ trợ tiếng Việt (vi) cho các bề mặt hướng người dùng để DS2API thân thiện hơn với lập trình viên Việt Nam.

What's new / Có gì mới

• Docs
  • README.vi.md – full Vietnamese mirror of README.MD
  • docs/README.vi.md – Vietnamese docs index
  • docs/DEPLOY.vi.md – concise local-first deployment guide in Vietnamese
  • Cross-links added to README.MD, README.en.md, docs/README.md, docs/DEPLOY.md, docs/DEPLOY.en.md so users can switch languages
• Config templates
  • Translated comments in config.example.json and .env.example to Vietnamese (no key/value structure changes)
• WebUI / Admin Console
  • New locale webui/src/locales/vi.json – 1:1 with zh.json (489 keys)
  • webui/src/i18n.jsx now exposes vi and falls back safely (vi → en → original key)
  • LanguageToggle.jsx cycles VI → EN → ZH
  • Added language.vietnamese label to existing en.json and zh.json
• Repo hygiene
  • .gitignore extended to ignore *.exe, .env.*.production, run.bat and accidental nested clones (no behavior change to authors' workflow)

Notes for reviewer / Ghi chú cho người review

• I kept zh as a fallback for users who have explicitly chosen Chinese in their localStorage. The default for new visitors whose browser is not vi / zh / en is vi; you can flip the constant DEFAULT_LANG in webui/src/i18n.jsx if you'd rather keep zh as the global default.
• No backend code (internal/, api/, cmd/) was touched – translations are limited to docs, config templates and the WebUI locale layer.
• npm run build --prefix webui succeeds locally (1774 modules transformed); admin assets continue to ship from static/admin/.

How to test / Cách kiểm thử

go run ./cmd/ds2api
# Open http://127.0.0.1:5001/admin → UI loads in Vietnamese by default
# Click language toggle → cycles VI → EN → ZH

Happy to adjust the default language behavior, copy or scope per maintainer preference.

Sẵn lòng điều chỉnh ngôn ngữ mặc định, văn phong hoặc phạm vi theo ý của tác giả.

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