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.dom.test.ts

@@ -129,6 +129,22 @@ describe("useRowSelection", () => {
     expect(result.current.shownSelected).toEqual(["c"]);
   });
 
+  it("counts the selected rows a filter hides", () => {
+    const { result, rerender } = setup();
+    act(() => result.current.toggle("a"));
+    act(() => result.current.toggle("c"));
+    expect(result.current.hiddenCount).toBe(0);
+    rerender({ all: ABCDE, shown: ["c", "d"], scope: 1 });
+    expect(result.current.hiddenCount).toBe(1);
+  });
+
+  it("ticks rows again on top of what is selected by then", () => {
+    const { result } = setup();
+    act(() => result.current.toggle("e"));
+    act(() => result.current.select(["a", "b"]));
+    expect(result.current.shownSelected).toEqual(["a", "b", "e"]);
+  });
+
   it("says whether clearing did anything", () => {
     const { result } = setup();
     let cleared = true;