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)

app/proprietary/src/main/java/stirling/software/proprietary/controller/api/ProprietaryUIDataController.java

@@ -205,6 +205,10 @@ public class ProprietaryUIDataController {
         data.setLoginMethod(securityProps.getLoginMethod());
         data.setAltLogin(!providerList.isEmpty() && securityProps.isAltLogin());
 
+        // Add language configuration for login page
+        data.setLanguages(applicationProperties.getUi().getLanguages());
+        data.setDefaultLocale(applicationProperties.getSystem().getDefaultLocale());
+
         return ResponseEntity.ok(data);
     }
 
@@ -492,6 +496,8 @@ public class ProprietaryUIDataController {
         private boolean altLogin;
         private boolean firstTimeSetup;
         private boolean showDefaultCredentials;
+        private List<String> languages;
+        private String defaultLocale;
     }
 
     @Data