fleet-manager: go fix — strings.Split -> strings.SplitSeq in newWebAuthn

`go fix` modernised the comma-origin loop to the Go 1.24+ iterator idiom
(strings.SplitSeq). No behaviour change.

serving/fleet-manager/webauthn.go

@@ -196,7 +196,7 @@ func newWebAuthn(cfg config) (*webauthn.WebAuthn, error) {
 	// the direct :9500 form and a 443-proxy form — since passkeys bind to the
 	// exact scheme://host:port the browser used. Each must match a real origin.
 	var origins []string
-	for _, o := range strings.Split(cfg.rpOrigin, ",") {
+	for o := range strings.SplitSeq(cfg.rpOrigin, ",") {
 		if o = strings.TrimSpace(o); o != "" {
 			origins = append(origins, o)
 		}