feat(i18n): Complete internationalization with timezone fixes and language switching

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.

proxy.ts

@@ -1,12 +1,7 @@
-import createMiddleware from 'next-intl/middleware';
-import { locales, defaultLocale } from './i18n/request';
+import createIntlMiddleware from 'next-intl/middleware';
+import { routing } from './i18n/routing';
 
-export default createMiddleware({
-  locales,
-  defaultLocale,
-  localePrefix: 'always', // Always show locale in URL for consistency
-  localeDetection: true // Enable browser language detection
-});
+export default createIntlMiddleware(routing);
 
 export const config = {
   // Skip all paths that should not be internationalized