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.select.dom.test.tsx

@@ -10,9 +10,15 @@ import { useActiveScopes } from "./keymap";
 
 afterEach(cleanup);
 
-function Rows({ selection }: { selection?: NavSelection }) {
+function Rows({
+  selection,
+  onDelete,
+}: {
+  selection?: NavSelection;
+  onDelete?: (index: number) => void;
+}) {
   const ref = useRef<HTMLDivElement>(null);
-  useListNav(ref, { enabled: true, selection });
+  useListNav(ref, { enabled: true, selection, onDelete });
   return (
     <div ref={ref}>
       {[0, 1, 2].map((i) => (
@@ -39,6 +45,7 @@ function press(key: string, init: KeyboardEventInit = {}): KeyboardEvent {
 const selection = (cleared = true) => ({
   toggle: vi.fn<(index: number) => void>(),
   clear: vi.fn(() => cleared),
+  removeSelected: vi.fn(() => false),
 });
 
 describe("useListNav · selecting", () => {
@@ -108,6 +115,25 @@ describe("useListNav · selecting", () => {
     expect(idle.clear).toHaveBeenCalledTimes(1);
   });
 
+  it("has Delete go to the selection first, wherever the focus is", () => {
+    const s = selection();
+    s.removeSelected.mockReturnValue(true);
+    const onDelete = vi.fn();
+    render(<Rows selection={s} onDelete={onDelete} />);
+    expect(press("Delete").defaultPrevented).toBe(true);
+    expect(s.removeSelected).toHaveBeenCalledTimes(1);
+    expect(onDelete).not.toHaveBeenCalled();
+  });
+
+  it("has Delete go on to the walked row when there is no selection", () => {
+    const s = selection();
+    const onDelete = vi.fn();
+    render(<Rows selection={s} onDelete={onDelete} />);
+    press("j");
+    press("Delete");
+    expect(onDelete).toHaveBeenCalledExactlyOnceWith(0);
+  });
+
   it("leaves both keys alone in a list that takes no selection", () => {
     render(<Rows />);
     press("j");