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>

tests/test_api_mobile.py

@@ -467,6 +467,42 @@ class TestWhoAmI:
             assert data["email"] == _OWNER
             assert data["avatar_url"] is not None  # Gravatar URL
             assert data["is_admin"] is False
+            assert data["preferred_language"] is None  # not set yet
+        finally:
+            _cleanup(app)
+
+    def test_whoami_returns_preferred_language(self, mob_engine, mob_session):
+        """preferred_language from UserProfile is included in the whoami response."""
+        from app.main import app
+        from app.models import UserProfile
+
+        profile = UserProfile(
+            user_id=_OWNER,
+            display_name="Bob Test",
+            preferred_language="de",
+        )
+        mob_session.add(profile)
+        mob_session.commit()
+
+        client = _make_client(mob_engine)
+        try:
+            resp = client.get("/api/mobile/whoami")
+            assert resp.status_code == 200
+            data = resp.json()
+            assert data["preferred_language"] == "de"
+        finally:
+            _cleanup(app)
+
+    def test_whoami_no_profile_preferred_language_is_null(self, mob_engine):
+        """preferred_language is null when no UserProfile exists."""
+        from app.main import app
+
+        client = _make_client(mob_engine)
+        try:
+            resp = client.get("/api/mobile/whoami")
+            assert resp.status_code == 200
+            data = resp.json()
+            assert data["preferred_language"] is None
         finally:
             _cleanup(app)