Allow sorting the datagrid on small screens

Short description of what this feature will allow to do:

On screens below 768px, the EasyAdmin theme hides the datagrid header
(assets/css/easyadmin-theme/datagrids.css, @media (max-width: 767px)).
The column sort links live only in that header, so on phones there is no way
to sort the listing: the only options are changing the default sort in the CRUD
controller or editing the URL by hand. A "Sort" button next to the other index
page actions, visible only on small screens, could open a modal listing the
sortable columns and toggling their sort direction, reusing the same URLs as the
header links.

Example of how to use this feature

Nothing to configure: the button shows up on the index page of any CRUD
controller that has sortable fields, and only below 768px.

Start with assets/css/easyadmin-theme/datagrids.css and the @media rule that hides the datagrid header below 768px. Then trace the index page actions and existing header sort links to determine how the proposed small-screen Sort control can reuse their URLs; it is done when sortable CRUD listings expose sorting below 768px without changing larger-screen behavior.