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/lib/listNav.ts

@@ -45,6 +45,9 @@ interface NavList {
 export interface NavSelection {
   readonly toggle: (index: number) => void;
   readonly clear: () => boolean;
+  /** Delete: takes the selection out, wherever the focus is. False when there is none, and Delete
+   *  goes on to the item under the keyboard. */
+  readonly removeSelected: () => boolean;
 }
 
 const lists = new Map<HTMLElement, NavList>();
@@ -432,6 +435,12 @@ function onKey(e: KeyboardEvent): void {
       // On the item root, like Enter — and only in a list that registered a remover.
       remove: (ev) => {
         const cur = current();
+        for (const list of ordered()) {
+          if (!list.selection()?.removeSelected()) continue;
+          ev.preventDefault();
+          if (cur) refocusSlot(cur.list, cur.index, cur.item);
+          return;
+        }
         const remove = cur?.list.remove();
         if (!cur || !remove || document.activeElement !== cur.item) return;
         ev.preventDefault();