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.

i18n/routing.ts

@@ -0,0 +1,11 @@
+import { defineRouting } from 'next-intl/routing';
+
+export const routing = defineRouting({
+  locales: ['en', 'fr'],
+  defaultLocale: 'en',
+  localePrefix: 'never'
+});
+
+export const locales = routing.locales;
+export const defaultLocale = routing.defaultLocale;
+export type Locale = (typeof locales)[number];