Add SearXNG as configurable search provider

[jaycollett]

Summary

This PR adds SearXNG as a search provider option with user-configurable instance URL support.

SearXNG is a privacy-respecting, self-hosted metasearch engine. Unlike Google/Bing/DuckDuckGo which have fixed URLs, SearXNG instances are self-hosted with user-specific URLs. This implementation:

• Adds SearXNG to the search providers list
• Adds a new "SearXNG Instance URL" setting in Miscellaneous settings
• Dynamically injects the user's configured URL when performing searches
• Supports autocomplete via SearXNG's x-suggestions format

Changes

File	Change
storage/app/searchproviders.yaml	Add SearXNG provider entry
database/seeders/SettingsSeeder.php	Add searx_url setting (ID 15), add to provider options
app/Search.php	Modify providerDetails() to inject user's configured URL
lang/en/app.php	Add translation strings for new setting and provider

User Workflow

1. Settings → Homepage Search = Yes
2. Settings → SearXNG Instance URL = https://your-searx-instance.com
3. Settings → Default Search Provider = SearXNG
4. Search queries redirect to https://your-searx-instance.com/search?q=query

Testing

• Tested locally with Docker using linuxserver/heimdall image
• Verified search redirects correctly to configured SearXNG instance
• Verified autocomplete functionality works with SearXNG's x-suggestions endpoint

[jaycollett]

Closing this PR in favor of implementing SearXNG as a proper search provider app in the Heimdall-Apps repo, following the standard SearchInterface pattern.

[jaycollett]

Reopening this PR after investigating the alternative approach of implementing SearXNG as a SearchInterface app in Heimdall-Apps.

Why the Heimdall-Apps approach doesn't work:

The Item::isSearchProvider() method in app/Item.php uses class_exists($class, false) which disables PHP autoloading. This means app-based search providers (like Whoogle, Searx, NZBHydra) don't appear in the search dropdown unless the class has already been loaded by some other means.

Why this PR's approach is appropriate:

Since SearXNG requires a user-configurable URL (unlike fixed providers like Google/Bing), adding it to searchproviders.yaml with special handling in Search.php is a reasonable solution that:

1. Makes SearXNG immediately available in the dropdown
2. Allows users to configure their instance URL in settings
3. Works around the autoloading bug in isSearchProvider()

A separate fix for the class_exists($class, false) issue is probably warrented?