[6.x] Make the whole radio/checkbox item clickable

[duncanmcclean]

This pull request makes the entire radio and checkbox item — including the padding and empty space around the control and label — clickable, rather than only the control and label text.

Previously the clickable area was limited to the Reka control (CheckboxRoot / RadioGroupItem) and the <label> wrapping the label text. This meant clicking the surrounding padding of a chip (or the description) did nothing.

This PR fixes it by stretching the inner <label>'s hit-area across the whole item using an absolutely-positioned before pseudo-element (before:absolute before:inset-0), with the container made relative to anchor it. The label keeps its for/id association to the control, so clicks anywhere in the item forward a single activation to the checkbox/radio.

This PR also tidies up the radio item's accessibility by giving the description an id and wiring it to the control via aria-describedby, matching the checkbox component.