Add Vietnamese localization end-to-end and harden email translation fallback

[Copilot]

This change adds Vietnamese (vi) to the localization surface and ensures the app remains functional when backend email templates are not yet translated for a newly added language. Scope covers UI/guide/privacy content, language models, and email-template resolution behavior.

• Frontend localization surface

  • Added src/lib/translations/vi.ts with Vietnamese translations for UI flows, guides, and privacy content.
  • Registered vi in src/lib/translations/index.ts.
  • Extended frontend language model/options in src/lib/types.ts (Language + LANGUAGES entry for Vietnamese).

• Backend language model + parity

  • Extended API Language union in api/src/shared/types.ts to include vi (and keep parity with existing frontend language set).

• Email localization robustness

  • Updated email template lookup in api/src/shared/email-service.ts to use a centralized fallback resolver, so unsupported/new languages resolve to English template content instead of relying on per-template assumptions.

• Translation docs

  • Updated src/lib/translations/README.md language list to include Vietnamese.

• Focused regression coverage

  • Added API test in api/src/__tests__/email-service.test.ts validating Vietnamese email generation falls back to English content deterministically.

function getTranslation<T>(translations: Partial<Record<Language, T>>, language: Language): T {
  const translation = translations[language] ?? translations.en
  if (!translation) {
    throw new Error(`English email translation is missing for language "${language}"`)
  }
  return translation
}

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[Copilot]

Pull request overview

This PR adds Vietnamese (vi) as a first-class language across the frontend and backend type models, and hardens backend email template resolution so newly-added/unsupported languages reliably fall back to English rather than producing missing-template behavior.

Changes:

• Added full Vietnamese UI/guide/privacy translations and registered vi in the frontend translation registry.
• Extended both frontend and API Language unions (and frontend LANGUAGES options) to include Vietnamese.
• Centralized backend email template translation lookup via a getTranslation helper (English fallback) and added a regression test covering vi fallback behavior.

Show a summary per file

File	Description
src/lib/types.ts	Adds vi to the frontend Language union and language picker options.
src/lib/translations/vi.ts	Introduces Vietnamese translation dictionary for the UI and guide/privacy content.
src/lib/translations/README.md	Documents Vietnamese as an available translation file.
src/lib/translations/index.ts	Registers vi in the exported translations map.
api/src/shared/types.ts	Extends backend Language union to include vi (and keep parity with frontend).
api/src/shared/email-service.ts	Adds a centralized translation resolver with English fallback and uses it across email generators.
api/src/__tests__/email-service.test.ts	Adds a unit test asserting Vietnamese email generation falls back to English deterministically.

Copilot's findings

• Files reviewed: 7/7 changed files
• Comments generated: 1

[github-actions]

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