feat(cluster): add default container registry field to advanced config

Surface the top-level system_default_container_registry as a dedicated
input (both Docker and K8s), coerce blank to null, and drop its
redundant worker_config YAML hint.

src/pages/cluster-management/components/cluster-form.tsx

@@ -64,8 +64,18 @@ const ClusterForm: React.FC<AddModalProps> = forwardRef(
     // "no auth". Coerce empty form values to null before sending so a blank
     // input is unambiguous rather than an empty string that defeats fallbacks.
     const normalizeOutgoing = (values: any): any => {
-      const opts = values?.k8s_options;
-      if (!opts) return values;
+      const base: any = { ...values };
+      // Top-level cluster field shared by Docker and K8s. Trim then coerce a
+      // blank input to null so clearing it on edit (or a whitespace-only
+      // value) falls back to the server default rather than persisting an
+      // empty string.
+      if (base.system_default_container_registry !== undefined) {
+        base.system_default_container_registry =
+          base.system_default_container_registry?.trim() || null;
+      }
+
+      const opts = base.k8s_options;
+      if (!opts) return base;
 
       const next: any = { ...opts };
 
@@ -90,7 +100,7 @@ const ClusterForm: React.FC<AddModalProps> = forwardRef(
         };
       }
 
-      return { ...values, k8s_options: next };
+      return { ...base, k8s_options: next };
     };
 
     const handleOnFinish = (_values: FormData) => {