Add SearXNG search provider app

[jaycollett]

Summary

Adds SearXNG as a search provider app implementing the SearchInterface.

• SearXNG is a privacy-respecting, self-hosted metasearch engine
• Actively maintained fork of the original Searx project
• Implements SearchInterface to enable search provider functionality

Files Added

File	Description
SearXNG/SearXNG.php	SearchInterface implementation
SearXNG/app.json	App metadata
SearXNG/searxng.png	Official SearXNG icon

Usage

1. Add SearXNG as an app tile in Heimdall
2. Configure the URL to point to your SearXNG instance
3. SearXNG will then appear as an available search provider

References

• SearXNG Documentation: https://docs.searxng.org
• SearXNG GitHub: https://github.com/searxng/searxng

[jaycollett]

Closing this PR - discovered that the SearchInterface approach has a bug in Heimdall core where class_exists($class, false) disables autoloading, preventing app-based search providers from appearing in the dropdown. Reverting to the original approach of adding SearXNG to searchproviders.yaml in the main Heimdall repo.