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


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

frontend/src/core/components/shared/AppConfigLoader.tsx

@@ -14,8 +14,9 @@ export default function AppConfigLoader() {
 
   useEffect(() => {
     if (!loading && config) {
-      // Update supported languages if config specifies a language filter
-      updateSupportedLanguages(config.languages);
+      // Update supported languages and apply default locale from server config
+      // Priority: localStorage > config.defaultLocale > browser detection > fallback
+      updateSupportedLanguages(config.languages, config.defaultLocale);
     }
   }, [config, loading]);