feat(playlists): select rows with a box, Shift for a range, x and Esc

A box first in every row and one over the column for the rows shown, the
selected row tinted. A filter keeps what it hides selected; another list
starts over; a row that left the list drops out. Watch later has none.

A box no longer counts as a typing target: a click on one left the focus
there and the page's keys went dead.

frontend/src/components/ui/tableHeader.tsx

@@ -232,7 +232,9 @@ export function ColumnHead<T>({
                     two of them per playlist table, whose grip and actions headers are empty. */}
                 {/* min-w-6 as well as min-h-6: a short header ("Ki", Hungarian "Who") was a 12px
                     target beside its funnel, under the 24px floor. */}
-                {col.sortable ? (
+                {col.headerCell ? (
+                  col.headerCell
+                ) : col.sortable ? (
                   <Pressable
                     onClick={() => onToggleSort(col.key)}
                     className="inline-flex min-h-6 min-w-6 items-center gap-1 hover:text-fg cursor-pointer touch-target"