feat(seo): add canonical URL filter to remove query parameters

Addresses community feedback about canonical URLs being misleading
when they include UTM parameters. The new Pongo2 filter creates
clean canonical URLs while preserving tracking parameters for
social sharing.

bskyweb/cmd/bskyweb/filters.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"net/url"
+
+	"github.com/flosch/pongo2/v6"
+)
+
+func init() {
+	pongo2.RegisterFilter("canonical", filterCanonical)
+}
+
+func filterCanonical(in *pongo2.Value, param *pongo2.Value) (*pongo2.Value, *pongo2.Error) {
+	urlStr := in.String()
+
+	parsedURL, err := url.Parse(urlStr)
+	if err != nil {
+		// If parsing fails, return the original URL
+		return in, nil
+	}
+
+	// Remove query parameters and fragment
+	parsedURL.RawQuery = ""
+	parsedURL.Fragment = ""
+
+	// Return the cleaned URL
+	return pongo2.AsValue(parsedURL.String()), nil
+}