fix: a huge export or list page answers an empty page instead of panicking (WR-14)

(page-1)*perPage overflowed for a large page and sliced out of range.
Export compares the page with the page count before multiplying, and the
list endpoints cap the page number.

classes/transfer.go

@@ -160,9 +160,11 @@ func (s *Service) Export(ctx context.Context, dataset, delimiter string, page, p
 			page = 1
 		}
 		out.Page = page
-		start := (page - 1) * perPage
-		if start > len(rows) {
-			start = len(rows)
+		// A page past the last is empty; compared before multiplying, so a
+		// huge page cannot overflow into a negative start (WR-14).
+		start := len(rows)
+		if page <= out.Pages {
+			start = (page - 1) * perPage
 		}
 		end := min(start+perPage, len(rows))
 		rows = rows[start:end]