Add: Configurable UI Language Support with Dynamic Filtering

[Ludy87]

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.

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Checklist

General

• I have read the Contribution Guidelines
• I have read the Stirling-PDF Developer Guide (if applicable)
• I have read the How to add new languages to Stirling-PDF (if applicable)
• I have performed a self-review of my own code
• My changes generate no new warnings

Documentation

• I have updated relevant docs on Stirling-PDF's doc repo (if functionality has heavily changed)
• I have read the section 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)

• I have tested my changes locally. Refer to the Testing Guide for more details.