feat(playlists): act on the selected rows from the toolbar, and undo either

While rows are selected the toolbar's second row is the band: the count,
the last target as one click, Move to, Remove, Clear selection. Both write
in one request and undo back to the places; a video the target held before
stays there. Delete takes the selection when there is one. A failure puts
the rows back ticked.

A row's own move goes through the same write, so it undoes too, and the
single-row move route and service function are gone. A playlist row drops
its grip below a 24rem table: the row menu keeps the moves.

frontend/src/components/ui/RowSelectBox.tsx

@@ -1,3 +1,4 @@
+import type { RefObject } from "react";
 import { Checkbox } from "./NativeControls";
 
 /** A table row's selecting box, and the same box over the column for all the rows. The whole cell is
@@ -9,7 +10,9 @@ export function RowSelectBox({
   mixed,
   title,
   onToggle,
+  ref,
 }: {
+  ref?: RefObject<HTMLInputElement | null>;
   label: string;
   checked: boolean;
   mixed?: boolean;
@@ -26,6 +29,7 @@ export function RowSelectBox({
       className="-ml-2 grid min-h-6 w-9 cursor-pointer place-items-center touch-target"
     >
       <Checkbox
+        ref={ref}
         aria-label={label}
         checked={checked}
         mixed={mixed}