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/useRowSelection.ts

@@ -59,8 +59,14 @@ export function useRowSelection(all: readonly string[], shown: readonly string[]
     return had;
   }
 
+  /** Tick these again: an action that took them off and failed. */
+  function select(ids: readonly string[]) {
+    setSelected((now) => new Set([...now, ...ids]));
+  }
+
   return {
     shownSelected,
+    hiddenCount: selected.size - shownSelected.length,
     isSelected: (id: string) => selected.has(id),
     headerState:
       shownSelected.length === 0
@@ -71,5 +77,6 @@ export function useRowSelection(all: readonly string[], shown: readonly string[]
     toggle,
     toggleAllShown,
     clear,
+    select,
   };
 }