feat: Proxmox nested nodes — VM/LXC rendered inside resizable group container

- ProxmoxGroupNode: resizable bordered container with orange accent, header
  bar (icon, label, IP, status dot), NodeResizer on selection
- nodeTypes: proxmox now uses ProxmoxGroupNode instead of BaseNode
- NodeData: added parent_id field
- canvasStore: addNode and loadCanvas wire parentId + extent='parent'
  for child nodes; loadCanvas sorts parents before children (RF requirement)
- NodeModal: Parent Proxmox dropdown appears when type is VM or LXC,
  lists all Proxmox nodes on the canvas
- App.tsx: passes proxmoxNodes to modals; child nodes positioned relative
  to parent (x:20, y:50); Proxmox nodes get width:300/height:200 on load
- layout.ts: Dagre skips child nodes (parentId set); uses actual group
  dimensions for Proxmox nodes in rank calculation

frontend/src/types/index.ts

@@ -39,6 +39,7 @@ export interface NodeData extends Record<string, unknown> {
   last_seen?: string
   response_time_ms?: number
   notes?: string
+  parent_id?: string
 }
 
 export interface EdgeData extends Record<string, unknown> {