fix(network): persist group node type, size, and child parentId on save/load

Backend DiagramNode schema was missing nodeType, style, and parentId fields —
Pydantic stripped them on save, so group nodes lost their identity on reload
and re-appeared as small device icons.

- Backend: add nodeType, style (NodeStyle), parentId to DiagramNode schema
- Frontend: serialize parentId for device nodes inside groups
- Frontend: restore parentId + extent:'parent' on both deserializer paths (setNodes + history init)
- Frontend: add parentId to DiagramNode interface

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/types/network-diagram.ts

@@ -18,6 +18,7 @@ export interface DiagramNode {
   properties: DeviceProperties
   nodeType?: string
   style?: { width?: number; height?: number } | null
+  parentId?: string | null
 }
 
 export interface DiagramEdge {