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/navigation.ts

@@ -0,0 +1,4 @@
+import { createNavigation } from 'next-intl/navigation';
+import { routing } from './routing';
+
+export const { Link, redirect, usePathname, useRouter } = createNavigation(routing);

i18n/request.ts

@@ -1,16 +1,11 @@
 import { getRequestConfig } from 'next-intl/server';
-
-export const locales = ['en', 'fr'] as const;
-export type Locale = (typeof locales)[number];
-export const defaultLocale: Locale = 'en';
+import { routing, type Locale } from './routing';
 
 export default getRequestConfig(async ({ requestLocale }) => {
-  // Get the locale from the request or use default
-  let locale = await requestLocale || defaultLocale;
+  let locale = await requestLocale;
 
-  // Validate that the incoming `locale` parameter is valid
-  if (!(locales as readonly string[]).includes(locale)) {
-    locale = defaultLocale;
+  if (!locale || !routing.locales.includes(locale as Locale)) {
+    locale = routing.defaultLocale;
   }
 
   // Use static imports for better compatibility

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];