fix(favorites): use cursor-based pagination instead of page numbers

FA's /favorites/{user}/ pagination is cursor-addressed by the fave-ID
of the last item on the previous page (e.g.
/favorites/{user}/1951234825/next), not by sequential integers. The
previous URL builder generated /favorites/{user}/{N}/ for N>=2; FA
interpreted that as a malformed cursor and silently returned page 1,
which caused the Favorites iterator to loop forever and the new
FavoritesPage to report HasNext=true on every call.

Changes:
- urls.Favorites(name) returns the first-page URL; new
  urls.FavoritesCursor(name, cursor) builds /favorites/.../next URLs.
- FavoritesPage now takes a cursor string; empty = first page.
  Returns ListingPage.NextPage as the opaque fave-ID for the next call.
- ListingPage gains NextPage string (decimal page number for
  Gallery/Scraps, fave-ID cursor for Favorites) and drops the Page int
  field that conflated those two notions.
- Client.Favorites iterator now walks cursors internally; StartPage
  is ignored for favorites (documented).
- detectNextPage / nextPageURL now parse the form action so the same
  helper works for both page-number and cursor pagination.
- Added regression test that fails on the infinite-loop bug.
- Example: examples/favorites_page demonstrates cursor walking.

examples/favorites_page/main.go

@@ -0,0 +1,79 @@
+// favorites_page exercises the per-page favorites listing API
+// ([Client.FavoritesPage]) against the live FA site so a caller can see
+// exactly what fields come back: HasNext, NextPage, len(Items), and a
+// sample of the tag data lifted from each figure's data-tags attribute.
+//
+// Favorites pagination is cursor-based: each page returns an opaque
+// NextPage token that addresses the next page. Pass it back in on the
+// next call; treat empty as end-of-pagination.
+//
+// Required environment variables:
+//
+//	FA_A           — the `a` session cookie
+//	FA_B           — the `b` session cookie
+//	CF_CLEARANCE   — (optional) cf_clearance cookie if Cloudflare challenges
+//	FA_UA          — (optional) User-Agent matching CF_CLEARANCE
+//
+// Usage:
+//
+//	go run ./examples/favorites_page <username> [maxPages]
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"os"
+	"strconv"
+
+	fa "git.anthrove.art/public/go-fa-api"
+)
+
+func main() {
+	if len(os.Args) < 2 {
+		log.Fatalf("usage: %s <username> [maxPages]", os.Args[0])
+	}
+	user := os.Args[1]
+	maxPages := 0
+	if len(os.Args) >= 3 {
+		if n, err := strconv.Atoi(os.Args[2]); err == nil && n > 0 {
+			maxPages = n
+		}
+	}
+
+	opts := []fa.Option{
+		fa.WithCookies(fa.Cookies{A: os.Getenv("FA_A"), B: os.Getenv("FA_B")}),
+	}
+	if cf := os.Getenv("CF_CLEARANCE"); cf != "" {
+		opts = append(opts, fa.WithCloudflare(fa.CFCookies{Clearance: cf}))
+	}
+	if ua := os.Getenv("FA_UA"); ua != "" {
+		opts = append(opts, fa.WithUserAgent(ua))
+	}
+	client := fa.New(opts...)
+
+	cursor := ""
+	pageNum := 0
+	for {
+		pageNum++
+		lp, err := client.FavoritesPage(context.Background(), user, cursor)
+		if err != nil {
+			log.Fatalf("FavoritesPage(cursor=%q): %v", cursor, err)
+		}
+		fmt.Printf("=== page %d  cursor=%q  items=%d  HasNext=%v  NextPage=%q ===\n",
+			pageNum, cursor, len(lp.Items), lp.HasNext, lp.NextPage)
+		for i, sub := range lp.Items {
+			fmt.Printf("  [%d] id=%d  rating=%s  author=%s  title=%q\n",
+				i, sub.ID, sub.Rating, sub.Author.Name, sub.Title)
+		}
+		if !lp.HasNext {
+			fmt.Printf("\nreached end of pagination after %d page(s)\n", pageNum)
+			return
+		}
+		if maxPages > 0 && pageNum >= maxPages {
+			fmt.Printf("\nstopped at maxPages=%d (HasNext was still true; next cursor=%q)\n", maxPages, lp.NextPage)
+			return
+		}
+		cursor = lp.NextPage
+	}
+}