feat(market): embed offer photos inline instead of a media server

Replace the Blossom media-server approach with a fully decentralized one:
generate a small (~320px) JPEG thumbnail from the lot's cover photo and
embed it in the offer event as a data: URI, so photos ride the relays with
no server and no IP leak. The full photo stays in the encrypted inventory.

- offer_thumbnail.dart: downscale-until-it-fits data-URI builder + decoder
- market_widgets: render data: URIs from memory, http(s) URLs from network
- offers_cubit: publish a thumbnail (best-effort) in place of an upload
- drop MediaTransport/Blossom from commons_core (http/crypto deps) and the
  media-server setting; parked on branch parked/offer-photos-blossom

Relay event-size limits cap the image to a thumbnail; higher-res sharing
would need the parked Blossom path.

apps/app_seeds/lib/state/offers_cubit.dart

@@ -8,6 +8,7 @@ import 'package:flutter_bloc/flutter_bloc.dart';
 import '../data/variety_repository.dart';
 import '../services/offer_mapper.dart';
 import '../services/offer_outbox.dart';
+import '../services/offer_thumbnail.dart';
 import '../services/social_service.dart';
 import '../services/social_settings.dart';
 
@@ -137,24 +138,24 @@ class OffersState extends Equatable {
 class OffersCubit extends Cubit<OffersState> {
   OffersCubit(
     this._transport, {
-    MediaTransport? media,
     Future<Uint8List?> Function(String varietyId)? coverPhoto,
+    String? Function(Uint8List bytes)? thumbnail,
     Future<void> Function()? onDispose,
-  })  : _media = media,
-        _coverPhoto = coverPhoto,
+  })  : _coverPhoto = coverPhoto,
+        _thumbnail = thumbnail,
         _onDispose = onDispose,
         super(const OffersState());
 
   final OfferTransport? _transport;
 
-  /// Hosts an offer's cover photo so peers can see it; null when no media server
-  /// is configured, in which case offers publish without images.
-  final MediaTransport? _media;
-
-  /// Fetches a variety's cover photo bytes (for hosting); null in tests or when
-  /// no inventory repo is wired.
+  /// Fetches a variety's cover photo bytes; null in tests or when no inventory
+  /// repo is wired.
   final Future<Uint8List?> Function(String varietyId)? _coverPhoto;
 
+  /// Turns full photo bytes into a small `data:` thumbnail embedded in the offer
+  /// (no media server). Null → offers publish without a photo.
+  final String? Function(Uint8List bytes)? _thumbnail;
+
   /// Closes the owning [SocialSession]/connection when the cubit is disposed
   /// (null in tests, where the transport is a fake with nothing to close).
   final Future<void> Function()? _onDispose;
@@ -284,7 +285,7 @@ class OffersCubit extends Cubit<OffersState> {
           areaGeohash: areaGeohash,
           category: lot.category,
           isOrganic: lot.isOrganic,
-          imageUrl: await _hostCoverPhoto(lot.varietyId),
+          imageUrl: await _coverThumbnail(lot.varietyId),
         );
         final result = await transport.publish(offer);
         if (result.accepted) accepted++;
@@ -297,18 +298,17 @@ class OffersCubit extends Cubit<OffersState> {
     return accepted;
   }
 
-  /// Uploads the lot's cover photo to the media server and returns its URL, or
-  /// null when there's no media server, no photo, or the upload fails. Hosting
-  /// is best-effort: a missing image never blocks publishing the offer.
-  Future<String?> _hostCoverPhoto(String varietyId) async {
-    final media = _media;
+  /// Builds a small inline `data:` thumbnail from the lot's cover photo to embed
+  /// in the offer, or null when there's no photo, no thumbnailer, or it can't be
+  /// shrunk to fit. Best-effort: a missing image never blocks publishing.
+  Future<String?> _coverThumbnail(String varietyId) async {
     final coverPhoto = _coverPhoto;
-    if (media == null || coverPhoto == null) return null;
+    final thumbnail = _thumbnail;
+    if (coverPhoto == null || thumbnail == null) return null;
     try {
       final Uint8List? bytes = await coverPhoto(varietyId);
       if (bytes == null || bytes.isEmpty) return null;
-      final url = await media.upload(bytes, mimeType: 'image/jpeg');
-      return url.toString();
+      return thumbnail(bytes);
     } catch (_) {
       return null; // degrade: publish the offer without a photo
     }
@@ -334,13 +334,11 @@ Future<OffersCubit> createOffersCubit(
   final relays = await settings.relayUrls();
   if (relays.isEmpty) return OffersCubit(null);
   try {
-    final mediaServer = await settings.mediaServerUrl();
-    final session =
-        await social.openSession(relays, mediaServerUrl: mediaServer);
+    final session = await social.openSession(relays);
     return OffersCubit(
       session.offers,
-      media: session.media,
       coverPhoto: repository?.coverPhotoFor,
+      thumbnail: offerThumbnailDataUri,
       onDispose: session.close,
     );
   } catch (_) {