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/core/src/main/java/stirling/software/SPDF/config/InitialSetup.java

@@ -58,6 +58,7 @@ public class InitialSetup {
             applicationProperties.getAutomaticallyGenerated().setKey(secretKey);
         }
     }
+
     public void initLegalUrls() throws IOException {
         // Initialize Terms and Conditions
         String termsUrl = applicationProperties.getLegal().getTermsAndConditions();

app/core/src/main/java/stirling/software/SPDF/controller/api/misc/ConfigController.java

@@ -74,6 +74,7 @@ public class ConfigController {
             configData.put("appNameNavbar", applicationProperties.getUi().getAppNameNavbar());
             configData.put("languages", applicationProperties.getUi().getLanguages());
             configData.put("logoStyle", applicationProperties.getUi().getLogoStyle());
+            configData.put("defaultLocale", applicationProperties.getSystem().getDefaultLocale());
 
             // Security settings
             // enableLogin requires both the config flag AND proprietary features to be loaded