MangasListState previously stored selected manga IDs as List<int>.
Every visible library card called .contains() on that list once per
rebuild to determine its highlight state, making each check O(n) in
the number of selected items. The provider's own update/selectAll/
selectSome methods also used .contains() and .remove() on a List.
Change the state type to Set<int> throughout, making all membership
checks O(1). Updated all consumers: library_gridview_widget,
library_listview_widget, library_app_bar, library_dialogs, and
MangasSetIsReadState.