perf(market): paginate Nostr offer discovery, infinite scroll, memory cap

Bound market memory and let large areas page instead of loading everything:
- DiscoveryQuery gains an until cursor; OfferTransport gains discoverPage()
  (one-shot, EOSE-bounded, newest-first) alongside the existing live discover()
  stream, which now accepts since so it only carries NEW offers going forward.
- NostrOfferTransport.discoverPage sorts by created_at desc and derives the
  next cursor from the oldest event seen (not the oldest type-matched one, so
  filtering never skips a page).
- OffersCubit: discover() fetches the first page + opens a since=now live sub;
  loadNextPage() pages further back; the in-memory list is capped at 400
  offers (each can carry a ~40KB inline photo thumbnail, so unbounded growth in
  a busy area was a real OOM risk).
- market_screen: infinite scroll via a scroll-position trigger + footer
  spinner, instead of a single unbounded ListView.
- Added discoverPage unit tests (commons_core) and cubit pagination tests
  (first page/cursor, accumulation, cap, cross-page dedup).

apps/app_seeds/test/screenshots/screenshots_test.dart

@@ -156,7 +156,11 @@ class _SeededOffersCubit extends OffersCubit {
 /// A transport that is present (so the market reads as online) but never used.
 class _NoopOfferTransport implements OfferTransport {
   @override
-  Stream<Offer> discover(DiscoveryQuery query) => const Stream.empty();
+  Stream<Offer> discover(DiscoveryQuery query, {int? since}) =>
+      const Stream.empty();
+  @override
+  Future<OfferPage> discoverPage(DiscoveryQuery query) async =>
+      const OfferPage(offers: []);
   @override
   Future<PublishResult> publish(Offer offer) => throw UnimplementedError();
   @override