Wrap the element in a fieldset and modernize its UI

Problem/Motivation

The group has no real legend and relies on a Webform-only label attribute. On phones the operations and handle are hidden. The icons use fixed fills that vanish on dark themes, Enter in a row input triggers the first row button, and two forms share a table id. The stylesheet follows the viewport rather than the container, colors and sizes are hard coded, and add and remove are image inputs.