feat(listing): add per-page methods with HasNext flag

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.

pagination.go

@@ -6,6 +6,24 @@ import (
 	"github.com/PuerkitoBio/goquery"
 )
 
+// ListingPage is one page of a listing endpoint (Gallery / Scraps /
+// Favorites). It carries everything an external caller needs to drive
+// pagination by hand: the items, the 1-based page number that produced
+// them, and whether FA exposed a "next page" link.
+//
+// External scrapers that want to manage their own loop (resume from a
+// checkpoint, run pages in parallel, throttle differently) should call
+// the per-page methods ([Client.GalleryPage], [Client.ScrapsPage],
+// [Client.FavoritesPage]) and stop when HasNext is false. Callers that
+// just want every item in order should keep using the iter.Seq2-shaped
+// methods ([Client.Gallery] et al.), which use the same primitive
+// internally.
+type ListingPage struct {
+	Items   []*Submission
+	HasNext bool
+	Page    int // 1-based page number this result corresponds to
+}
+
 // ListOptions configures the pagination of a simple iterator method like
 // [Client.Gallery] or [Client.Notes]. Filtered iterators ([Client.Search],
 // [Client.Browse]) use their own option structs that fold the same fields