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/modals/__tests__/NodeModal.test.tsx

@@ -237,7 +237,7 @@ describe('NodeModal', () => {
   // ── Container mode ─────────────────────────────────────────────────────
 
   const containerModeTypes = ['proxmox', 'vm', 'lxc', 'docker_host'] as const
-  const nonContainerModeTypes = ['isp', 'router', 'switch', 'server', 'nas', 'ap', 'printer', 'iot', 'camera', 'cpl', 'computer', 'generic', 'groupRect', 'group'] as const
+  const nonContainerModeTypes = ['isp', 'router', 'switch', 'server', 'nas', 'ap', 'printer', 'iot', 'camera', 'cpl', 'computer', 'generic', 'docker_container', 'groupRect', 'group'] as const
 
   it.each(containerModeTypes)('shows Container Mode toggle for %s type', (type) => {
     renderModal({ initial: { ...BASE, type } })
@@ -280,6 +280,26 @@ describe('NodeModal', () => {
     expect(screen.queryByText('Parent Container')).toBeNull()
   })
 
+  it('docker_container shows only docker_host parents', () => {
+    renderModal({
+      initial: { ...BASE, type: 'docker_container' },
+      parentContainerNodes: [
+        { id: 'h1', label: 'My Docker Host', nodeType: 'docker_host' },
+        { id: 'p1', label: 'My Proxmox', nodeType: 'proxmox' },
+      ],
+    })
+    expect(screen.getByText('My Docker Host')).toBeDefined()
+    expect(screen.queryByText('My Proxmox')).toBeNull()
+  })
+
+  it('docker_container hides Parent Container when no docker_host is available', () => {
+    renderModal({
+      initial: { ...BASE, type: 'docker_container' },
+      parentContainerNodes: [{ id: 'p1', label: 'My Proxmox', nodeType: 'proxmox' }],
+    })
+    expect(screen.queryByText('Parent Container')).toBeNull()
+  })
+
   // ── Appearance ────────────────────────────────────────────────────────
 
   it('renders 3 color swatch labels (border, background, icon)', () => {