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/resources/settings.yml.template

@@ -101,6 +101,7 @@ ui:
   appName: '' # application's visible name
   homeDescription: '' # short description or tagline shown on the homepage
   appNameNavbar: '' # name displayed on the navigation bar
+  languages: [] # If empty, all languages ​​are enabled. To display only German and Polish ["de_DE", "pl_PL"]. British English is always enabled.
 
 endpoints:
   toRemove: [] # list endpoints to disable (e.g. ['img-to-pdf', 'remove-pages'])