Add: Configurable UI Language Support with Dynamic Filtering (#2846)

# Description of Changes

### Summary
- Added support for configuring UI languages via `settings.yml`
(`languages` field).
- Modified `LanguageService` to respect the configured languages, while
ensuring British English (`en_GB`) is always enabled.
- Updated Thymeleaf templates to dynamically display only the allowed
languages.
- Improved logging and refactored some list-to-set conversions for
better efficiency.

### Why the Change?
- Allows administrators to limit available UI languages instead of
displaying all detected languages.
- Provides better customization options and simplifies language
management.

### Challenges Encountered
- Ensuring backwards compatibility: If `languages` is empty, all
languages remain enabled.
- Handling `Set<String>` instead of `List<String>` in `LanguageService`
for optimized lookups.

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [x] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [x] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [x] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#6-testing)
for more details.

src/main/java/stirling/software/SPDF/model/ApplicationProperties.java

@@ -265,7 +265,8 @@ public class ApplicationProperties {
                             return getKeycloak();
                         default:
                             throw new UnsupportedProviderException(
-                                    "Logout from the provider is not supported? Report it at https://github.com/Stirling-Tools/Stirling-PDF/issues");
+                                    "Logout from the provider is not supported? Report it at"
+                                            + " https://github.com/Stirling-Tools/Stirling-PDF/issues");
                     }
                 }
             }
@@ -313,10 +314,10 @@ public class ApplicationProperties {
         @Override
         public String toString() {
             return """
-                    Driver {
-                      driverName='%s'
-                    }
-                    """
+            Driver {
+              driverName='%s'
+            }
+            """
                     .formatted(driverName);
         }
     }
@@ -326,6 +327,7 @@ public class ApplicationProperties {
         private String appName;
         private String homeDescription;
         private String appNameNavbar;
+        private List<String> languages;
 
         public String getAppName() {
             return appName != null && appName.trim().length() > 0 ? appName : null;