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/components/canvas/nodes/index.tsx

@@ -1,7 +1,7 @@
 import { type NodeProps, type Node } from '@xyflow/react'
 import {
   Globe, Router, Network, Server, Layers, Box, Container,
-  HardDrive, Cpu, Wifi, Circle, Cctv, Printer, Monitor, PlugZap, Anchor,
+  HardDrive, Cpu, Wifi, Circle, Cctv, Printer, Monitor, PlugZap, Anchor, Package,
 } from 'lucide-react'
 import { BaseNode } from './BaseNode'
 import type { NodeData } from '@/types'
@@ -22,5 +22,6 @@ export const CameraNode   = (props: N) => <BaseNode {...props} icon={Cctv} />
 export const PrinterNode  = (props: N) => <BaseNode {...props} icon={Printer} />
 export const ComputerNode = (props: N) => <BaseNode {...props} icon={Monitor} />
 export const CplNode      = (props: N) => <BaseNode {...props} icon={PlugZap} />
-export const DockerHostNode = (props: N) => <BaseNode {...props} icon={Anchor} />
+export const DockerHostNode      = (props: N) => <BaseNode {...props} icon={Anchor} />
+export const DockerContainerNode = (props: N) => <BaseNode {...props} icon={Package} />
 export const GenericNode  = (props: N) => <BaseNode {...props} icon={Circle} />