DTO - UI Data speration

Server gives you `{foo: string}` and you want `{foo: string, expanded: boolean}` etc. I need to document the guidance for this.

Anybody got a nice heading for this? :rose:

The issue names no file, test, or entry point. Review the book's existing guidance for server data and UI models, then document how to add UI-only fields such as `expanded` to `{foo: string}`; done means the guidance has a clear heading and explains this separation.

Written by the indexing model from the issue text.