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/test/services/social_settings_test.dart

@@ -27,15 +27,6 @@ void main() {
     expect(await settings.relayUrls(), isEmpty);
   });
 
-  test('media server defaults, is configurable, and can be turned off',
-      () async {
-    expect(await settings.mediaServerUrl(), SocialSettings.defaultMediaServer);
-    await settings.setMediaServerUrl('  https://blossom.example  ');
-    expect(await settings.mediaServerUrl(), 'https://blossom.example');
-    await settings.setMediaServerUrl(''); // explicit empty = no photo hosting
-    expect(await settings.mediaServerUrl(), '');
-  });
-
   test('stores and normalises the area geohash (trim + lowercase)', () async {
     await settings.setAreaGeohash('  SP3E9  ');
     expect(await settings.areaGeohash(), 'sp3e9');