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/service/LanguageService.java

@@ -1,41 +1,57 @@
 package stirling.software.SPDF.service;
 
 import java.io.IOException;
-import java.util.ArrayList;
-import java.util.List;
+import java.util.HashSet;
+import java.util.Set;
+import java.util.stream.Collectors;
+import java.util.Arrays;
 
 import org.springframework.core.io.Resource;
 import org.springframework.core.io.support.PathMatchingResourcePatternResolver;
 import org.springframework.stereotype.Service;
 
+import lombok.extern.slf4j.Slf4j;
+import stirling.software.SPDF.model.ApplicationProperties;
+
 @Service
+@Slf4j
 public class LanguageService {
 
+    private final ApplicationProperties applicationProperties;
     private final PathMatchingResourcePatternResolver resourcePatternResolver =
             new PathMatchingResourcePatternResolver();
 
-    public List<String> getSupportedLanguages() {
-        List<String> supportedLanguages = new ArrayList<>();
+    public LanguageService(
+            ApplicationProperties applicationProperties) {
+        this.applicationProperties = applicationProperties;
+    }
 
+    public Set<String> getSupportedLanguages() {
         try {
             Resource[] resources =
                     resourcePatternResolver.getResources("classpath*:messages_*.properties");
-            for (Resource resource : resources) {
-                if (resource.exists() && resource.isReadable()) {
-                    String filename = resource.getFilename();
-                    if (filename != null
-                            && filename.startsWith("messages_")
-                            && filename.endsWith(".properties")) {
-                        String languageCode =
-                                filename.replace("messages_", "").replace(".properties", "");
-                        supportedLanguages.add(languageCode);
-                    }
-                }
-            }
-        } catch (IOException e) {
-            e.printStackTrace();
-        }
 
-        return supportedLanguages;
+            return Arrays.stream(resources)
+                    .map(Resource::getFilename)
+                    .filter(
+                            filename ->
+                                    filename != null
+                                            && filename.startsWith("messages_")
+                                            && filename.endsWith(".properties"))
+                    .map(filename -> filename.replace("messages_", "").replace(".properties", ""))
+                    .filter(
+                            languageCode -> {
+                                Set<String> allowedLanguages =
+                                        new HashSet<>(applicationProperties.getUi().getLanguages());
+                                return allowedLanguages.isEmpty()
+                                        || allowedLanguages.contains(languageCode)
+                                        || "en_GB".equals(languageCode);
+                            })
+                    .collect(Collectors.toSet());
+
+        } catch (IOException e) {
+            log.error("Error retrieving supported languages", e);
+            return new HashSet<>();
+        }
     }
 }