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/services/social_service.dart

@@ -39,34 +39,20 @@ class SocialService {
   /// Opens a session against [relayUrls]: one fault-tolerant pool carrying all
   /// three transports (offers, messaging, trust). Relays that are down are
   /// skipped; throws only when none are reachable. Caller closes it.
-  ///
-  /// [mediaServerUrl] (a Blossom endpoint) enables offer photos: when set, the
-  /// session exposes a [MediaTransport] to host images. It rides HTTP, not the
-  /// relay pool, so it's independent of relay reachability.
-  Future<SocialSession> openSession(
-    List<String> relayUrls, {
-    String? mediaServerUrl,
-  }) async {
+  Future<SocialSession> openSession(List<String> relayUrls) async {
     final pool = await RelayPool.connect(relayUrls, identity: identity);
-    return SocialSession(pool, mediaServerUrl: mediaServerUrl);
+    return SocialSession(pool);
   }
 }
 
 /// One relay channel with the three transports on top — the
-/// "one channel, three interfaces" shape, at the app layer. Media rides its own
-/// HTTP endpoint (Blossom), so it's a fourth transport but not on the channel.
+/// "one channel, three interfaces" shape, at the app layer.
 class SocialSession {
-  SocialSession(this._channel, {String? mediaServerUrl})
+  SocialSession(this._channel)
       : offers = NostrOfferTransport(_channel),
         messages = NostrMessageTransport(_channel),
         trust = NostrTrustTransport(_channel),
-        profile = NostrProfileTransport(_channel),
-        media = (mediaServerUrl != null && mediaServerUrl.trim().isNotEmpty)
-            ? BlossomMediaTransport(
-                Uri.parse(mediaServerUrl.trim()),
-                _channel.privateKeyHex,
-              )
-            : null;
+        profile = NostrProfileTransport(_channel);
 
   final NostrChannel _channel;
 
@@ -75,14 +61,7 @@ class SocialSession {
   final TrustTransport trust;
   final ProfileTransport profile;
 
-  /// Hosts offer photos, or null when no media server is configured (offers then
-  /// publish without images).
-  final MediaTransport? media;
-
-  Future<void> close() async {
-    await media?.close();
-    await _channel.close();
-  }
+  Future<void> close() => _channel.close();
 }
 
 Uint8List _hexToBytes(String hex) {