Add dynu_root_domain variable for domain import flow

infrastructure/terraform/dynu/domains.tf

@@ -1,3 +1,9 @@
 locals {
-  dynu_domain = "lan.ddnsgeek.com"
+  dynu_domain = var.dynu_root_domain
+}
+
+# Import-first resource skeleton for the production Dynu zone.
+# `name` is required by provider schema and can be reconciled after import.
+resource "dynu_domain" "lan_ddnsgeek_com" {
+  name = local.dynu_domain
 }