test(auth): add regression coverage for i18n across the auth flow (#159)

[datvt243]

Commits going into staging

• 9d4b69a test(auth): add regression coverage for i18n across the auth flow (i18n for API messages — auth flow (phase 1/2) #159)

Summary

The i18n infrastructure (utils/i18n.ts, locales/{vi,en}.ts, language.middleware.ts) and the full auth-flow migration (register/login/logout/refresh/forgot-password/reset-password/verify-email) were already implemented and live — all 21 message sites already resolve through t(). This adds the missing regression tests: unit tests for t()/tErrorType(), unit tests for languageMiddleware's Accept-Language resolution, and 5 new English-language cases at the auth-controller boundary proving req.lang drives real localized output. 0 production code changed.

Closes #159.

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