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/stores/canvasStore.ts

@@ -297,7 +297,6 @@ export const useCanvasStore = create<CanvasState>((set) => ({
       let nodes = state.nodes.map((n) => {
         if (n.id === proxmoxId) {
           const withMode = { ...n, data: { ...n.data, container_mode: enabled } }
-          if (n.data.type !== 'proxmox') return withMode
           return enabled
             ? { ...withMode, width: n.width ?? 300, height: n.height ?? 200 }
             : { ...withMode, width: undefined, height: undefined }