Laterite admin: the operator-facing web surface.
An Axum router mounted at /admin: a login screen and session cookie
verified against laterite-auth, and descriptor-driven screens.
Screens are resources: a module declares a [Resource] (a
[list::ListConfig], optionally a [form::FormConfig], a base path, and a
menu label), and the framework mounts the list, create, and edit routes and
adds it to the menu. This is the extension point that lets an application
contribute its own admin screens. The framework's own screens (users, roles)
are just built-in resources.