feat(sort): give SortControl the FilterSelect pill look with a footer direction segment

Rebuilds SortControl's internals (API unchanged, so all five consumers — feed, Plex,
playlists, and the two table/card ColumnSortControl sites — get it for free) as a labeled
"Label: key ↓ ▾" pill on the AnchoredPopover platform, matching FilterSelect. The listbox
lists the sort keys; an asc/desc segment sits in the panel footer (with the trailing slot —
feed reshuffle, playlists dirty-first — beside it).

Two-dimensional key+direction, so it stays its own component rather than reusing FilterSelect:
the trigger is a single plain button (no nested button), picking a key keeps the panel open so
key AND direction can be set in one visit, and picking "no sort" (tables' defaultLabel) closes
it. Directionless keys (shuffle) hide the segment and drop the trigger arrow.

Drops the now-redundant external "Sort" label spans in Feed and PlexBrowse (the trigger carries
the label now). Rewrites SortControl.dom.test for the popover UI.

frontend/src/components/Feed.tsx

@@ -626,7 +626,6 @@ export default function Feed({
             : " "}
         </span>
         <div className="ml-auto flex items-center gap-2">
-          <span className="text-xs text-muted">{t("feed.sortLabel")}</span>
           <SortControl
             options={sortOptions}
             sort={{ key: sortKey, dir: sortDir }}