feat: add OpenSearch XML endpoint

- Serve /opensearch.xml with configurable base URL
- Browsers can now add gosearch as a search engine from the address bar
- Configurable via [server] base_url or BASE_URL env var
- XML template embedded in the binary via go:embed
- Added base_url to config.example.toml

config.example.toml

@@ -9,6 +9,11 @@ port = 8080
 # HTTP timeout for engine and upstream calls (env: HTTP_TIMEOUT)
 http_timeout = "10s"
 
+# Public base URL for OpenSearch XML (env: BASE_URL)
+# Set this so browsers can add gosearch as a search engine.
+# Example: "https://search.example.com"
+base_url = ""
+
 [upstream]
 # URL of an upstream SearXNG instance for unported engines (env: UPSTREAM_SEARXNG_URL)
 # Leave empty to run without an upstream proxy.