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.

components/providers/intl-provider.tsx

@@ -0,0 +1,63 @@
+"use client";
+
+import { useEffect, useState } from 'react';
+import { NextIntlClientProvider } from 'next-intl';
+import { useLocaleStore } from '@/stores/locale-store';
+import enMessages from '@/locales/en/common.json';
+import frMessages from '@/locales/fr/common.json';
+
+// Pre-loaded translations (loaded at build time, not runtime)
+const ALL_MESSAGES = {
+  en: enMessages,
+  fr: frMessages,
+};
+
+interface IntlProviderProps {
+  locale: string;
+  messages: Record<string, unknown>;
+  children: React.ReactNode;
+}
+
+export function IntlProvider({ locale: initialLocale, children }: IntlProviderProps) {
+  const currentLocale = useLocaleStore((state) => state.locale);
+  const setLocale = useLocaleStore((state) => state.setLocale);
+  const [activeLocale, setActiveLocale] = useState(currentLocale || initialLocale);
+  const [timeZone, setTimeZone] = useState<string>('UTC');
+
+  // Detect user's timezone on mount
+  useEffect(() => {
+    try {
+      const detectedTimeZone = Intl.DateTimeFormat().resolvedOptions().timeZone;
+      setTimeZone(detectedTimeZone);
+    } catch (error) {
+      // Fallback to UTC if detection fails
+      console.warn('Failed to detect timezone, using UTC:', error);
+      setTimeZone('UTC');
+    }
+  }, []);
+
+  // Sync initial locale with store on first mount only
+  useEffect(() => {
+    if (!currentLocale) {
+      setLocale(initialLocale);
+    }
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, []);
+
+  // Switch locale immediately when store changes
+  useEffect(() => {
+    if (currentLocale) {
+      setActiveLocale(currentLocale);
+    }
+  }, [currentLocale]);
+
+  return (
+    <NextIntlClientProvider
+      locale={activeLocale}
+      messages={ALL_MESSAGES[activeLocale as keyof typeof ALL_MESSAGES]}
+      timeZone={timeZone}
+    >
+      {children}
+    </NextIntlClientProvider>
+  );
+}