Handle restricted language configuration fallback V2 (#5154)

## Summary
- restrict supported languages to the validated list from app-config
instead of always adding an extra fallback
- set the effective fallback locale to the preferred configured language
and switch away from disallowed selections automatically

## Testing
- ./gradlew build


------
[Codex
Task](https://chatgpt.com/codex/tasks/task_b_6930529bc6c08328a1ce05f7d1316e27)

frontend/src/core/components/AppProviders.tsx

@@ -20,6 +20,7 @@ import ErrorBoundary from "@app/components/shared/ErrorBoundary";
 import { useScarfTracking } from "@app/hooks/useScarfTracking";
 import { useAppInitialization } from "@app/hooks/useAppInitialization";
 import { useLogoAssets } from '@app/hooks/useLogoAssets';
+import AppConfigLoader from '@app/components/shared/AppConfigLoader';
 
 // Component to initialize scarf tracking (must be inside AppConfigProvider)
 function ScarfTrackingInitializer() {
@@ -81,6 +82,7 @@ export function AppProviders({ children, appConfigRetryOptions, appConfigProvide
                 {...appConfigProviderProps}
               >
                 <ScarfTrackingInitializer />
+                <AppConfigLoader />
                 <FileContextProvider enableUrlSync={true} enablePersistence={true}>
                   <AppInitializer />
                   <BrandingAssetManager />