Nested group hierarchy in the admin UI

## What

visualize and manage nested group hierarchy (children tab / tree view / subgroups in the user-folders panel) instead of a flat group list.

## Want

- Manage nested group hierarchy (children tab / tree view / subgroups in the user-folders panel) instead of a flat group list

## Ideas this came from

- [#3698 Add subgroups to 'User folders' panel](https://github.com/goauthentik/authentik/issues/3698) — It can be difficult to visualize group hierarchy, or to display users which are in particular groups on the main view, for users managed in Authentik. Add Authe
- [#15361 Feature Request: Improved Nested Group Management in authentik](https://github.com/goauthentik/authentik/issues/15361) — Managing nested groups in authentik becomes challenging and non-intuitive when dealing with a large number of groups, especially in collaborative environments.
- [#17670 Group children tab in the group details view](https://github.com/goauthentik/authentik/issues/17670) — Feature request, It helps answering questions fast about group hierarchy As the title says, just like there's a "Users" tab in the group details, the same for t
- [#17671 Group tree view (similar to the user path tree)](https://github.com/goauthentik/authentik/issues/17671) — The user tree view is a nice visualization that I'd like to see on the group side as well A similar control as the user tree, in which I could see all the "top-

Start by reading the referenced issues #3698, #15361, #17670, and #17671 to understand the proposed children tab, tree view, and user-folders changes. Then locate the admin UI's current flat group list and user-folders panel. Done means users can visualize and manage nested group hierarchies instead of seeing only a flat list.

Written by the indexing model from the issue text.