Add a custom admin field widget

1. Name the widget in model admin metadata:

admin: {
  displayName: "Articles",
  listFields: ["title", "body"],
  fieldOverrides: {body: {widget: "rich-text"}},
}

2. Implement the field widget:

const RichTextWidget: React.FC<AdminFieldWidgetProps> = ({
  value,
  onChange,
  readOnly,
}) => (
  <RichTextEditor
    disabled={Boolean(readOnly)}
    onChange={onChange}
    value={typeof value === "string" ? value : ""}
  />
);

3. Register it once around the admin routes:

<AdminProvider
  api={api}
  apiBase="/admin"
  routeBase="/admin"
  widgets={{fields: {"rich-text": RichTextWidget}}}
>
  <AdminRoutes />
</AdminProvider>

Host registrations override built-ins with the same ID.