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>

backend/app/schemas/network_diagram.py

@@ -22,12 +22,20 @@ class DeviceProperties(BaseModel):
     status: str = Field(default="unknown", pattern=r"^(unknown|online|offline|degraded)$")
 
 
+class NodeStyle(BaseModel):
+    width: float | None = None
+    height: float | None = None
+
+
 class DiagramNode(BaseModel):
     id: str
     type: str
     label: str
     position: Position
     properties: DeviceProperties = Field(default_factory=DeviceProperties)
+    nodeType: str | None = None
+    style: NodeStyle | None = None
+    parentId: str | None = None
 
 
 class DiagramEdge(BaseModel):