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/contexts/AppConfigContext.tsx

@@ -20,6 +20,7 @@ export interface AppConfig {
   serverPort?: number;
   appNameNavbar?: string;
   languages?: string[];
+  defaultLocale?: string;
   logoStyle?: 'modern' | 'classic';
   enableLogin?: boolean;
   enableEmailInvites?: boolean;