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/templates/opensearch.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/">
+  <ShortName>gosearch</ShortName>
+  <Description>A privacy-respecting, open metasearch engine</Description>
+  <InputEncoding>UTF-8</InputEncoding>
+  <OutputEncoding>UTF-8</OutputEncoding>
+  <LongName>gosearch — Privacy-respecting metasearch</LongName>
+  <Image width="16" height="16" type="image/svg+xml">/static/img/favicon.svg</Image>
+  <Contact>https://git.ashisgreat.xyz/penal-colony/gosearch</Contact>
+  <Url type="text/html" method="GET" template="{baseUrl}/search?q={searchTerms}&amp;format=html">
+    <Param name="pageno" value="{startPage?}" />
+    <Param name="language" value="{language?}" />
+  </Url>
+  <Url type="application/json" method="GET" template="{baseUrl}/search?q={searchTerms}&amp;format=json" />
+  <Query role="example" searchTerms="golang" />
+</OpenSearchDescription>

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{