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/settings/appearance-settings.tsx

@@ -3,6 +3,7 @@
 import { useTranslations } from 'next-intl';
 import { useThemeStore } from '@/stores/theme-store';
 import { useSettingsStore } from '@/stores/settings-store';
+import { LanguageSwitcher } from '@/components/ui/language-switcher';
 import { SettingsSection, SettingItem, RadioGroup, ToggleSwitch } from './settings-section';
 
 export function AppearanceSettings() {
@@ -25,6 +26,11 @@ export function AppearanceSettings() {
         />
       </SettingItem>
 
+      {/* Language */}
+      <SettingItem label={t('language.label')} description={t('language.description')}>
+        <LanguageSwitcher />
+      </SettingItem>
+
       {/* Font Size */}
       <SettingItem label={t('font_size.label')} description={t('font_size.description')}>
         <RadioGroup