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/utils/nodeIcons.ts

@@ -98,8 +98,8 @@ export const ICON_REGISTRY: IconEntry[] = [
   { key: 'radio',       label: 'MQTT / RTL-SDR',       category: 'Automation', icon: Radio },
 
   // --- Containers & Dev ---
-  { key: 'anchor',    label: 'Portainer / Docker',   category: 'Dev & Containers', icon: Anchor },
-  { key: 'package',   label: 'Docker Host',          category: 'Dev & Containers', icon: Package },
+  { key: 'anchor',    label: 'Portainer / Docker',        category: 'Dev & Containers', icon: Anchor },
+  { key: 'package',   label: 'Docker Host / Container',  category: 'Dev & Containers', icon: Package },
   { key: 'gitbranch', label: 'Gitea / Gitlab',       category: 'Dev & Containers', icon: GitBranch },
   { key: 'terminal',  label: 'SSH / Shell',          category: 'Dev & Containers', icon: Terminal },
   { key: 'code',      label: 'VS Code Server',       category: 'Dev & Containers', icon: Code2 },
@@ -132,8 +132,9 @@ export const NODE_TYPE_DEFAULT_ICONS: Record<NodeType, LucideIcon> = {
   printer:      Printer,
   computer:     Monitor,
   cpl:          PlugZap,
-  docker_host:  Anchor,
-  generic:      Circle,
+  docker_host:       Anchor,
+  docker_container:  Package,
+  generic:           Circle,
   group:        Circle,
   groupRect:    Circle,
 }