Add opt-in SSO authentication alongside Basic Auth. OAuth endpoints are
auto-discovered via .well-known, with support for external IdPs
(Keycloak, Authentik) via configurable OAUTH_ISSUER_URL. Sessions
persist through httpOnly refresh token cookies with automatic renewal.
Completes full internationalization coverage for the webmail application with enhanced language support:
- Replace all remaining hardcoded English strings with translation keys
- Add timezone auto-detection to prevent hydration mismatches
- Enable instant client-side language switching without page reload
- Add 60+ new translation keys in English and French locales
- Improve language switcher component with proper state management
- Add health check endpoint for container orchestration
All user-facing text now supports English and French with no hardcoded fallbacks, providing a fully localized experience.