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

internal/views/views.go

@@ -80,6 +80,18 @@ func StaticFS() (fs.FS, error) {
 	return fs.Sub(staticFS, "static")
 }
 
+// OpenSearchXML returns the OpenSearch description XML with {baseUrl}
+// replaced by the provided base URL.
+func OpenSearchXML(baseURL string) ([]byte, error) {
+	tmplFS, _ := fs.Sub(templatesFS, "templates")
+	data, err := fs.ReadFile(tmplFS, "opensearch.xml")
+	if err != nil {
+		return nil, err
+	}
+	result := strings.ReplaceAll(string(data), "{baseUrl}", baseURL)
+	return []byte(result), nil
+}
+
 // FromResponse builds PageData from a search response and request params.
 func FromResponse(resp contracts.SearchResponse, query string, pageno int) PageData {
 	pd := PageData{