GalleryPage / ScrapsPage / FavoritesPage return a ListingPage struct
carrying the page items, the 1-based page number, and a HasNext flag
that mirrors FA's "next page" link. This lets external scrapers drive
their own pagination loop (checkpoint resume, parallel workers,
custom throttling) without re-implementing the page-walking code.
The existing iter.Seq2-shaped methods now share the same per-page
primitive internally so behaviour stays in lock-step.
