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>

app/api/mobile.py

@@ -120,6 +120,7 @@ class WhoAmIResponse(BaseModel):
     email: str | None
     avatar_url: str | None
     is_admin: bool
+    preferred_language: str | None
 
 
 # ---------------------------------------------------------------------------
@@ -357,4 +358,5 @@ async def whoami(
         "email": email,
         "avatar_url": avatar_url,
         "is_admin": is_admin,
+        "preferred_language": profile.preferred_language if profile else None,
     }