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/controller/api/AdditionalLanguageJsController.java

@@ -2,7 +2,9 @@ package stirling.software.SPDF.controller.api;
 
 import java.io.IOException;
 import java.io.PrintWriter;
+import java.util.ArrayList;
 import java.util.List;
+import java.util.Set;
 
 import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.RequestMapping;
@@ -26,11 +28,11 @@ public class AdditionalLanguageJsController {
     @Hidden
     @GetMapping(value = "/additionalLanguageCode.js", produces = "application/javascript")
     public void generateAdditionalLanguageJs(HttpServletResponse response) throws IOException {
-        List<String> supportedLanguages = languageService.getSupportedLanguages();
+        Set<String> supportedLanguages = languageService.getSupportedLanguages();
         response.setContentType("application/javascript");
         PrintWriter writer = response.getWriter();
         // Erstelle das JavaScript dynamisch
-        writer.println("const supportedLanguages = " + toJsonArray(supportedLanguages) + ";");
+        writer.println("const supportedLanguages = " + toJsonArray(new ArrayList<>(supportedLanguages)) + ";");
         // Generiere die `getDetailedLanguageCode`-Funktion
         writer.println(
                 """

src/main/java/stirling/software/SPDF/controller/web/DatabaseWebController.java

@@ -36,8 +36,9 @@ public class DatabaseWebController {
         }
         List<FileInfo> backupList = databaseService.getBackupList();
         model.addAttribute("backupFiles", backupList);
-        model.addAttribute("databaseVersion", databaseService.getH2Version());
-        if ("Unknown".equalsIgnoreCase(databaseService.getH2Version())) {
+        String dbVersion = databaseService.getH2Version();
+        model.addAttribute("databaseVersion", dbVersion);
+        if ("Unknown".equalsIgnoreCase(dbVersion)) {
             model.addAttribute("infoMessage", "notSupported");
         }
         return "database";