feat: add localization support for en/pt/es/fr/de (#4)

Adds i18n infrastructure (thin custom + native Intl, no library) covering
auth pages, sidebar nav, language switcher, and common labels across the
five supported locales. English is canonical fallback at every layer.

- Schema: user_profiles.locale text column with named CHECK constraint
- Backend: PATCH /user/profile accepts/persists locale with allowlist validation
- Frontend: server-side locale resolution (cookie -> Accept-Language -> en)
  in root layout; client I18nProvider for runtime switching; ProfileLocaleSync
  applies authenticated profile.locale one-way
- Cookie 'mike_locale' (non-httpOnly, Path=/, Max-Age=1y, SameSite=Lax)
- Catalog validator with structural + placeholder + plural-shape checks
- 51 automated tests (46 frontend + 5 backend) on pure logic; React surface
  covered manually
- Coupled allowlist triple flagged: schema CHECK / backend SUPPORTED_LOCALES
  / frontend SupportedLocale must move together

Spec:  https://github.com/manueljpconde/mikeEU/issues/4#issuecomment-4414976968
Plan:  https://github.com/manueljpconde/mikeEU/issues/4#issuecomment-4414987693
GO:    https://github.com/manueljpconde/mikeEU/issues/4#issuecomment-4415157201

Closes #4