fix(mobile): wire i18n reactivity, translate all screens, sync language with server

- Add LocaleProvider + useLocale() hook with AsyncStorage persistence to mobile i18n
- Replace all hardcoded English strings in every screen with t() calls
- Add missing profile.settings/language keys to all 5 translation files (en/de/es/fr/it)
- Wrap app root in LocaleProvider; apply server preferred_language on login in AuthGuard
- Tab labels and header titles now re-render on language switch
- ProfileScreen: use useLocale() context, sync language to server via POST /api/i18n/language
- Backend: add preferred_language field to GET /api/mobile/whoami response
- Mobile API: add preferred_language to WhoAmIResponse type + setServerLanguage() method
- Tests: add test_whoami_returns_preferred_language and test_whoami_no_profile_preferred_language_is_null
- Docs: update MobileApp.md with language sync priority and whoami response format

Language priority: server preference > AsyncStorage > device locale > English fallback

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

mobile/src/i18n/de.json

@@ -99,7 +99,9 @@
     "delete_account_title": "Konto löschen",
     "delete_account_msg": "Dadurch werden Ihr Konto und alle zugehörigen Daten dauerhaft gelöscht. Diese Aktion kann nicht rückgängig gemacht werden.",
     "could_not_open": "Konnte {page} nicht öffnen. Bitte versuchen Sie es erneut.",
-    "admin": "Admin"
+    "admin": "Admin",
+    "settings": "Einstellungen",
+    "language": "Sprache"
   },
   "legal": {
     "privacy_policy": "Datenschutz",