fleet-manager: register-admin link uses the first of multiple RP origins

Follow-up to comma-separated RP_ORIGIN support: the enrolment URL builder used
the raw origin string, which would embed the whole comma list. Take the first
origin for the link.

serving/fleet-manager/cli.go

@@ -150,7 +150,13 @@ func runRegisterAdmin(args []string) int {
 		fmt.Fprintf(os.Stderr, "mint: %v\n", err)
 		return 1
 	}
-	base := strings.TrimRight(*origin, "/")
+	// RP_ORIGIN may list several origins (comma-separated); the enrolment link
+	// uses the first one.
+	base := *origin
+	if i := strings.IndexByte(base, ','); i >= 0 {
+		base = base[:i]
+	}
+	base = strings.TrimRight(strings.TrimSpace(base), "/")
 	if base == "" {
 		base = "https://<admin-ui-origin>"
 	}