feat: add SearXNG search provider with custom URL and JSON output

[norci]

Summary

Add SearXNG as a new search provider option for OpenFang.

Changes

• Added Searxng variant to SearchProvider enum
• Added SearxngSearchConfig struct with custom URL and max_results fields
• Implemented search_searxng method with JSON output format
• Added SearXNG to search_auto() fallback chain (tried after Perplexity, before DuckDuckGo)
• Added validation warning when SearXNG provider is selected but URL is not configured

Configuration

[web]
search_provider = "searxng"

[web.searxng]
url = "https://your-searxng-instance.com"
max_results = 5

Notes

• SearXNG is self-hosted, no API key required
• Output format: JSON with query, title, url, content fields
• Follows existing code patterns and passes clippy with -D warnings