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/offer_thumbnail_test.dart

@@ -0,0 +1,61 @@
+import 'dart:convert';
+import 'dart:typed_data';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:image/image.dart' as img;
+import 'package:tane/services/offer_thumbnail.dart';
+
+void main() {
+  /// A real, decodable photo far bigger than a thumbnail, with varied pixels so
+  /// JPEG can't compress it to nothing.
+  Uint8List bigPhoto() {
+    final image = img.Image(width: 1280, height: 960);
+    for (var y = 0; y < image.height; y++) {
+      for (var x = 0; x < image.width; x++) {
+        image.setPixelRgb(x, y, (x * 7) % 256, (y * 5) % 256, (x + y) % 256);
+      }
+    }
+    return img.encodeJpg(image, quality: 90);
+  }
+
+  test('produces a small jpeg data-URI that fits the byte budget', () {
+    final uri = offerThumbnailDataUri(bigPhoto(), maxBytes: 40000);
+
+    expect(uri, isNotNull);
+    expect(uri!, startsWith('data:image/jpeg;base64,'));
+    final b64 = uri.substring('data:image/jpeg;base64,'.length);
+    expect(b64.length, lessThanOrEqualTo(40000));
+
+    // The payload really is a smaller image than the source.
+    final decoded = img.decodeJpg(base64.decode(b64))!;
+    expect(decoded.width, lessThanOrEqualTo(320));
+    expect(decoded.height, lessThanOrEqualTo(320));
+  });
+
+  test('never upscales an already-tiny image', () {
+    final small = img.encodeJpg(img.Image(width: 64, height: 48), quality: 80);
+    final uri = offerThumbnailDataUri(small);
+
+    expect(uri, isNotNull);
+    final b64 = uri!.substring('data:image/jpeg;base64,'.length);
+    final decoded = img.decodeJpg(base64.decode(b64))!;
+    expect(decoded.width, 64);
+    expect(decoded.height, 48);
+  });
+
+  test('returns null for bytes that are not an image', () {
+    expect(offerThumbnailDataUri(Uint8List.fromList([1, 2, 3, 4])), isNull);
+  });
+
+  group('decodeDataUri', () {
+    test('round-trips a base64 data URI to its bytes', () {
+      final bytes = Uint8List.fromList([9, 8, 7, 6]);
+      final uri = 'data:image/jpeg;base64,${base64.encode(bytes)}';
+      expect(decodeDataUri(uri), bytes);
+    });
+
+    test('returns null for a plain http url', () {
+      expect(decodeDataUri('https://example.org/a.jpg'), isNull);
+    });
+  });
+}

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');