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>
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