feat(node-modal): restore Parent Container selector for child types

Adds back the Parent Container selector in NodeModal, shown when:
- child type is lxc or vm (parents: proxmox/vm/lxc/docker_host)
- child type is docker_container (parents: docker_host/lxc/vm/proxmox)

Selector hidden when no valid candidate node exists on the canvas.
Centralizes the parent-type allowlist in getValidParentTypes
(shared with the virtual-edge auto-parent helper).

ha-relevant: yes

frontend/src/utils/virtualEdgeParent.ts

@@ -15,6 +15,16 @@ export interface ParentAssignment {
   parentId: string
 }
 
+export function getValidParentTypes(childType: NodeType): NodeType[] {
+  if (childType === 'lxc' || childType === 'vm') {
+    return ['proxmox', 'vm', 'lxc', 'docker_host']
+  }
+  if (childType === 'docker_container') {
+    return ['docker_host', 'lxc', 'vm', 'proxmox']
+  }
+  return []
+}
+
 export function resolveVirtualEdgeParent(
   source: VirtualEdgeEndpoint,
   target: VirtualEdgeEndpoint,