<scroll-to-top>

Overlay button that appears once the page (or a given target container)
has scrolled more than threshold pixels away from the top edge, and
scrolls back to the top on click.

With no target (default), the button is position: fixed to the
viewport. When target is set, the button switches to `position:
absolute and expects to be placed as a descendant of a position:
relative` (or otherwise positioned) ancestor that establishes the visual
bounds to float within — typically target itself, given `overflow-y:
auto; position: relative`, so the button stays pinned to that container's
own visible corner as its content scrolls, rather than floating over the
whole page.

The control is styled as a standard secondary button (ui-button's
secondary variant: 2rem tall, --ui-radius-sm corners, the
--ui-button-secondary-* background/border tokens, so a gradient theme
carries over) rather than a pill, layered over an opaque --ui-surface
base and an elevation shadow, since it floats above scrolled content.