Let the filters bar reach the edges of the screen (#54266)

* Let the filters bar reach the edges of the screen

The row of filter and sort controls stopped at the safe area, so on a
device with a horizontal inset it sat in a narrower strip than the list
below it, its bottom border stopping short of the edge. The bar now
spans the full width, its controls scroll under the inset, and a spacer
at the end keeps the last one clear of it by the inset plus the usual
16px. A scrolled flex row drops its end padding, so that space has to be
an item rather than padding.

The subpage keeps the inset as padding inside its scroller instead of an
outer margin, so the bar can bleed into it, and a narrow data table
gives the header row the full width rather than clipping it. The rows of
the table itself stay inside the safe area as before.

* Address review on the filters bar safe area