feat(docker): allow LXC as parent for docker_container

Previously docker_container could only attach to a docker_host via
virtual edge. Extend the rule so an LXC node can also act as parent,
enabling docker_container nesting under LXC without requiring an
intermediate docker_host.

Extracts the virtual-edge parent rule into a pure helper
(resolveVirtualEdgeParent) with unit tests covering all type pairs.

ha-relevant: yes

frontend/src/utils/virtualEdgeParent.ts

@@ -0,0 +1,37 @@
+import type { NodeData } from '@/types'
+
+export type NodeType = NodeData['type']
+
+const CONTAINER_MODE_TYPES = new Set<NodeType>(['proxmox', 'vm', 'lxc', 'docker_host'])
+
+export interface VirtualEdgeEndpoint {
+  id: string
+  type: NodeType
+}
+
+export interface ParentAssignment {
+  childId: string
+  parentId: string
+}
+
+export function resolveVirtualEdgeParent(
+  source: VirtualEdgeEndpoint,
+  target: VirtualEdgeEndpoint,
+): ParentAssignment | null {
+  const { type: srcType, id: srcId } = source
+  const { type: tgtType, id: tgtId } = target
+
+  if ((srcType === 'lxc' || srcType === 'vm') && CONTAINER_MODE_TYPES.has(tgtType)) {
+    return { childId: srcId, parentId: tgtId }
+  }
+  if (CONTAINER_MODE_TYPES.has(srcType) && (tgtType === 'lxc' || tgtType === 'vm')) {
+    return { childId: tgtId, parentId: srcId }
+  }
+  if (srcType === 'docker_container' && (tgtType === 'docker_host' || tgtType === 'lxc')) {
+    return { childId: srcId, parentId: tgtId }
+  }
+  if (tgtType === 'docker_container' && (srcType === 'docker_host' || srcType === 'lxc')) {
+    return { childId: tgtId, parentId: srcId }
+  }
+  return null
+}