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",

mobile/src/i18n/en.json

@@ -99,7 +99,9 @@
     "delete_account_title": "Delete Account",
     "delete_account_msg": "This will permanently delete your account and all associated data. This action cannot be undone.",
     "could_not_open": "Could not open the {page}. Please try again.",
-    "admin": "Admin"
+    "admin": "Admin",
+    "settings": "Settings",
+    "language": "Language"
   },
   "legal": {
     "privacy_policy": "Privacy Policy",

mobile/src/i18n/es.json

@@ -99,7 +99,9 @@
     "delete_account_title": "Eliminar Cuenta",
     "delete_account_msg": "Esto eliminará permanentemente tu cuenta y todos los datos asociados. Esta acción no se puede deshacer.",
     "could_not_open": "No se pudo abrir {page}. Inténtalo de nuevo.",
-    "admin": "Admin"
+    "admin": "Admin",
+    "settings": "Configuración",
+    "language": "Idioma"
   },
   "legal": {
     "privacy_policy": "Privacidad",

mobile/src/i18n/fr.json

@@ -99,7 +99,9 @@
     "delete_account_title": "Supprimer le Compte",
     "delete_account_msg": "Cela supprimera définitivement votre compte et toutes les données associées. Cette action est irréversible.",
     "could_not_open": "Impossible d'ouvrir {page}. Veuillez réessayer.",
-    "admin": "Admin"
+    "admin": "Admin",
+    "settings": "Paramètres",
+    "language": "Langue"
   },
   "legal": {
     "privacy_policy": "Confidentialité",

mobile/src/i18n/index.ts

@@ -6,9 +6,23 @@
  * locales.
  *
  * Supported languages: English, German, Spanish, French, Italian.
+ *
+ * ## React integration
+ *
+ * Wrap the app root in `<LocaleProvider>` and call `useLocale()` in any
+ * component that renders translated strings.  `useLocale()` returns the
+ * active language code and a `setLang` setter that:
+ *   1. Updates the in-memory `currentLanguage` variable (so `t()` picks it up)
+ *   2. Triggers a React re-render of every consumer
+ *   3. Persists the choice to AsyncStorage (survives app restarts)
+ *
+ * Language priority on startup:
+ *   server preference (from /api/mobile/whoami) > AsyncStorage > device locale > "en"
  */
 
+import AsyncStorage from "@react-native-async-storage/async-storage";
 import { getLocales } from "expo-localization";
+import React from "react";
 
 import de from "./de.json";
 import en from "./en.json";
@@ -28,6 +42,8 @@ const translations: Record<string, TranslationMap> = { en, de, es, fr, it };
 // Locale detection
 // ---------------------------------------------------------------------------
 
+const LANG_STORAGE_KEY = "@docuelevate:language";
+
 /** Resolve the best-matching language code from the device locale list. */
 function detectLanguage(): string {
   try {
@@ -46,7 +62,7 @@ function detectLanguage(): string {
 let currentLanguage: string = detectLanguage();
 
 // ---------------------------------------------------------------------------
-// Public API
+// Plain-function public API (framework-agnostic)
 // ---------------------------------------------------------------------------
 
 /**
@@ -82,13 +98,22 @@ export function getLanguage(): string {
   return currentLanguage;
 }
 
-/** Override the language manually (e.g. from user settings). */
+/**
+ * Update the active language in memory.
+ * Prefer `useLocale().setLang` inside React components – it also persists
+ * the choice and triggers re-renders.
+ */
 export function setLanguage(lang: string): void {
   if (translations[lang]) {
     currentLanguage = lang;
   }
 }
 
+/** Return true if the given language code is supported by the mobile app. */
+export function isLanguageSupported(lang: string): boolean {
+  return Object.prototype.hasOwnProperty.call(translations, lang);
+}
+
 /** Return the list of supported language codes. */
 export function getSupportedLanguages(): { code: string; label: string }[] {
   return [
@@ -99,3 +124,85 @@ export function getSupportedLanguages(): { code: string; label: string }[] {
     { code: "it", label: "Italiano" },
   ];
 }
+
+// ---------------------------------------------------------------------------
+// React integration – context + provider + hook
+// ---------------------------------------------------------------------------
+
+interface LocaleContextValue {
+  /** The active language code, e.g. "en" or "de". */
+  lang: string;
+  /**
+   * Switch to a new language.  Persists the choice to AsyncStorage and
+   * triggers a re-render of every `useLocale()` consumer.
+   */
+  setLang: (code: string) => Promise<void>;
+}
+
+const LocaleContext = React.createContext<LocaleContextValue>({
+  lang: currentLanguage,
+  // Default setter used outside of a provider – updates in-memory only.
+  setLang: async (code: string) => {
+    setLanguage(code);
+  },
+});
+
+/**
+ * Wrap the app root in `LocaleProvider` to enable reactive language switching.
+ *
+ * On mount it reads the persisted language from AsyncStorage so the user's
+ * choice survives app restarts.  The server-preferred language is applied
+ * externally (see `AuthGuard` in `app/_layout.tsx`) after the profile is
+ * fetched from `/api/mobile/whoami`.
+ */
+export function LocaleProvider({ children }: { children: React.ReactNode }): React.ReactElement {
+  const [lang, setLangState] = React.useState(currentLanguage);
+
+  // Restore the persisted language preference once on app start.
+  React.useEffect(() => {
+    AsyncStorage.getItem(LANG_STORAGE_KEY)
+      .then((saved) => {
+        if (saved && isLanguageSupported(saved)) {
+          setLanguage(saved);
+          setLangState(saved);
+        }
+      })
+      .catch(() => {
+        // Ignore read errors – fall back to device-detected language.
+      });
+  }, []);
+
+  const setLang = React.useCallback(async (code: string): Promise<void> => {
+    if (!isLanguageSupported(code)) return;
+    setLanguage(code);
+    setLangState(code);
+    try {
+      await AsyncStorage.setItem(LANG_STORAGE_KEY, code);
+    } catch {
+      // Ignore write errors – the in-memory change is still applied.
+    }
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, []); // setLangState is a React state setter – its identity is guaranteed stable
+
+  const value = React.useMemo(() => ({ lang, setLang }), [lang, setLang]);
+
+  return React.createElement(LocaleContext.Provider, { value }, children);
+}
+
+/**
+ * Hook that subscribes to language changes.
+ *
+ * Any component calling `useLocale()` re-renders automatically when the
+ * language changes.  Call `t()` freely inside the component body – the
+ * re-render will pick up the new translations.
+ *
+ * ```tsx
+ * function MyScreen() {
+ *   const { lang, setLang } = useLocale(); // subscribes to changes
+ *   return <Text>{t("common.loading")}</Text>;
+ * }
+ * ```
+ */
+export function useLocale(): LocaleContextValue {
+  return React.useContext(LocaleContext);
+}

mobile/src/i18n/it.json

@@ -99,7 +99,9 @@
     "delete_account_title": "Elimina Account",
     "delete_account_msg": "Questo eliminerà permanentemente il tuo account e tutti i dati associati. Questa azione non può essere annullata.",
     "could_not_open": "Impossibile aprire {page}. Riprova.",
-    "admin": "Admin"
+    "admin": "Admin",
+    "settings": "Impostazioni",
+    "language": "Lingua"
   },
   "legal": {
     "privacy_policy": "Privacy",