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/PlexPlaylistView.menu.dom.test.tsx

@@ -152,14 +152,22 @@ describe("PlexPlaylistView · a row's menu", () => {
   it("removes a title with no confirm, and the toast's Undo puts it back at its index", async () => {
     mount([movie("1"), movie("2"), movie("3")]);
     await screen.findByText("Movie 2");
-    vi.spyOn(api, "plexPlaylistRemoveItem").mockResolvedValue({} as never);
+    vi.spyOn(api, "plexPlaylistRemoveItem").mockImplementation(async () => {
+      // The server's side of it: the Undo reads the list fresh, and the row is gone from it.
+      vi.mocked(api.plexPlaylist).mockResolvedValue({
+        id: 5,
+        title: "Films",
+        items: [movie("1"), movie("3")],
+      } as never);
+      return {} as never;
+    });
     const add = vi.spyOn(api, "plexPlaylistAddBulk").mockResolvedValue({} as never);
     await pick(() => fireEvent.contextMenu(rowOf("Movie 2")), "plex.playlist.removeFromPlaylist");
     await waitFor(() => expect(api.plexPlaylistRemoveItem).toHaveBeenCalledWith(5, "2"));
     expect(h.confirm).not.toHaveBeenCalled();
     await waitFor(() => expect(toast()?.message).toBe("plex.playlist.removedToast:Movie 2"));
     act(() => toast()!.action!.onClick());
-    expect(add).toHaveBeenCalledWith(5, ["2"], [1]);
+    await waitFor(() => expect(add).toHaveBeenCalledWith(5, ["2"], [1]));
   });
 
   it("plays and removes a whole show from its group's menu", async () => {
@@ -313,3 +321,24 @@ describe("PlexPlaylistView · the header's '…'", () => {
     );
   });
 });
+
+// E28 review E2: the watched toggle flips the view's LOCAL copy of the rows. On a failure the
+// resync refetches an identical list, react-query keeps the same `data`, and the copy was never
+// re-seeded — the row stayed "watched" on screen while Plex still had it unwatched.
+describe("PlexPlaylistView · a failed watched toggle", () => {
+  it("puts the row's watched state back", async () => {
+    mount([movie("1"), movie("2")]);
+    await screen.findByText("Movie 2");
+    vi.spyOn(api, "plexSetState").mockRejectedValue(new Error("offline"));
+    await pick(() => fireEvent.contextMenu(rowOf("Movie 2")), "plex.markWatched");
+    await waitFor(() =>
+      expect(h.notify).toHaveBeenCalledWith(
+        expect.objectContaining({ level: "error", title: "plex.watchedFailed" }),
+      ),
+    );
+    await act(async () => {});
+    fireEvent.keyDown(document.activeElement ?? document.body, { key: "Escape" });
+    await openMenu(() => fireEvent.contextMenu(rowOf("Movie 2")));
+    expect(menuRows()).toContain("plex.markWatched");
+  });
+});