feat: add docker_container node type and fix container mode for non-proxmox types

- Add docker_container node type (Package icon, sky-blue accent) as child of docker_host
- Parent selector for docker_container filters to docker_host only via nodeType field
- Virtual edge drag-connect syncs parent_id for docker_container <-> docker_host
- Fix setProxmoxContainerMode: remove proxmox-only guard so width/height are properly
  set/cleared for all container-capable types (docker_host, vm, lxc)
- Fix handleAddNode: only give group size when container_mode=true, making create
  and reload behavior consistent (was giving size unconditionally for CONTAINER_MODE_TYPES)
- Add regression tests for docker_host container mode toggle and docker_container nesting

frontend/src/types/index.ts

@@ -14,6 +14,7 @@ export type NodeType =
   | 'computer'
   | 'cpl'
   | 'docker_host'
+  | 'docker_container'
   | 'generic'
   | 'groupRect'
   | 'group'
@@ -128,6 +129,7 @@ export const NODE_TYPE_LABELS: Record<NodeType, string> = {
   computer: 'Computer',
   cpl: 'CPL / Powerline',
   docker_host: 'Docker Host',
+  docker_container: 'Docker Container',
   generic: 'Generic Device',
   groupRect: 'Group Rectangle',
   group: 'Node Group',