fix(playlists): an Undo puts a row back after its old neighbour, not at a stale index

A raw index recorded at removal drifted once anything changed before the Undo.
Both lists now read the list fresh and re-insert each row after the row that
preceded it (lib/reinsert). A failed watched toggle in a Plex playlist also
puts the row's state back: its local copy was never re-seeded.

frontend/src/components/Playlists.dom.test.tsx

@@ -449,9 +449,10 @@ describe("Playlists — removing a row, and undoing it", () => {
   });
 
   it("puts it back where it stood in the stored order, whatever the view on screen", async () => {
-    vi.spyOn(api, "removeFromPlaylist").mockResolvedValue(undefined as never);
+    const lists = { 1: [vid("A", 0), vid("B", 0), vid("C", 0)] };
+    serverRemoves(lists);
     vi.spyOn(api, "addToPlaylist").mockResolvedValue(undefined as never);
-    mount({ 1: [vid("A", 0), vid("B", 0), vid("C", 0)] });
+    mount(lists);
     await screen.findByRole("button", { name: "card.play — Video C" });
     // Sort the VIEW by title, descending: C, B, A on screen — the stored order is still A, B, C.
     fireEvent.click(screen.getByRole("button", { name: /playlists\.colTitle/ }));
@@ -463,10 +464,25 @@ describe("Playlists — removing a row, and undoing it", () => {
     await waitFor(() => expect(api.addToPlaylist).toHaveBeenCalledWith(1, "A", 0));
   });
 
+  // E28 review C4: the stored index drifts when the list changes before the Undo (a row added at
+  // the top in another tab) — the row goes back after its old neighbour instead.
+  it("puts it back after its neighbour when the list changed before the Undo", async () => {
+    const lists = { 1: [vid("A", 0), vid("B", 0), vid("C", 0)] };
+    serverRemoves(lists);
+    vi.spyOn(api, "addToPlaylist").mockResolvedValue(undefined as never);
+    mount(lists);
+    await rowMenu("Video B", "playlists.removeItem");
+    await waitFor(() => expect(lastToast()?.action).toBeTruthy());
+    lists[1] = [vid("X", 0), ...lists[1]!];
+    act(() => lastToast().action!.onClick());
+    await waitFor(() => expect(api.addToPlaylist).toHaveBeenCalledWith(1, "B", 2));
+  });
+
   it("says so when the Undo fails", async () => {
-    vi.spyOn(api, "removeFromPlaylist").mockResolvedValue(undefined as never);
+    const lists = { 1: [vid("A", 0), vid("B", 0)] };
+    serverRemoves(lists);
     vi.spyOn(api, "addToPlaylist").mockRejectedValue(new TypeError("Failed to fetch"));
-    mount({ 1: [vid("A", 0), vid("B", 0)] });
+    mount(lists);
     await rowMenu("Video A", "playlists.removeItem");
     await waitFor(() => expect(lastToast()?.action).toBeTruthy());
     act(() => lastToast().action!.onClick());