i18n: add Vietnamese (vi) translation

[miguelfernandezdev]

Translate VueWiki content to Vietnamese.

What to translate

• Question files: create docs/vi/q/ mirroring docs/q/
• UI labels: create docs/.vitepress/theme/locales/vi.json
• Keep code examples in English, translate surrounding text
• Keep the same frontmatter structure (translate only the title)

How to contribute

See CONTRIBUTING.md for the translation workflow. English is the source of truth.

You don't need to translate everything at once — start with a few questions and open a PR.

[miguelfernandezdev]

Update: UI locale file is already created at docs/.vitepress/theme/locales/. The UI labels (buttons, filters, flashcards, quiz, etc.) are translated and ready.

What's still needed:

• Question content files in docs/{lang}/q/ mirroring docs/q/
• VitePress locale config entry in docs/.vitepress/config.ts

A translation banner component is also in place — it will automatically show on pages that aren't yet translated, linking back to this issue.