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/httpapi/handlers.go

@@ -33,6 +33,19 @@ func (h *Handler) Index(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+// OpenSearch serves the OpenSearch description XML.
+func (h *Handler) OpenSearch(baseURL string) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		xml, err := views.OpenSearchXML(baseURL)
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+		w.Header().Set("Content-Type", "application/opensearchdescription+xml; charset=utf-8")
+		w.Write(xml)
+	}
+}
+
 func (h *Handler) Search(w http.ResponseWriter, r *http.Request) {
 	// For HTML format with no query, redirect to homepage.
 	if r.FormValue("q") == "" && (r.FormValue("format") == "" || r.FormValue("format") == "html") {