Refresh checkpoint UI (#294282)

* Refresh checkpoint and restore checkpoint UI

Replace bookmark icon and dashed divider with fading gradient lines flanking
the checkpoint/restore labels. Add dot separator and undo button styling for
the restore-checkpoint row. Use disabledForeground for label text with
hover-to-reveal border treatment on toolbar actions.

* Show checkpoint UI only on hover/focus, adjust spacing and colors

* Add aria-hidden to dot separator for screen readers