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/ui/market_widgets.dart

@@ -2,6 +2,7 @@ import 'package:commons_core/commons_core.dart';
 import 'package:flutter/material.dart';
 
 import '../i18n/strings.g.dart';
+import '../services/offer_thumbnail.dart';
 import 'theme.dart';
 
 /// Human words for an offer's reciprocity mode. Shared by the market list and
@@ -15,8 +16,9 @@ String offerTypeLabel(Translations t, OfferType type) => switch (type) {
     };
 
 /// A small square thumbnail of an offer's photo for the market list card.
-/// Remote (published) image, so it degrades gracefully: a spinner while it
-/// loads and a neutral placeholder when the media server can't be reached.
+/// The image usually rides inline in the offer as a `data:` URI (decentralized,
+/// no server), so it renders straight from memory; an `http(s)` URL is still
+/// supported for interop and degrades gracefully (spinner, then placeholder).
 class OfferThumbnail extends StatelessWidget {
   const OfferThumbnail({
     required this.url,
@@ -33,23 +35,18 @@ class OfferThumbnail extends StatelessWidget {
   Widget build(BuildContext context) {
     return ClipRRect(
       borderRadius: BorderRadius.circular(10),
-      child: Image.network(
+      child: _offerImage(
         url,
+        semanticLabel: semanticLabel,
         width: size,
         height: size,
-        fit: BoxFit.cover,
-        semanticLabel: semanticLabel,
-        loadingBuilder: (context, child, progress) => progress == null
-            ? child
-            : _OfferImagePlaceholder(size: size, loading: true),
-        errorBuilder: (context, _, _) => _OfferImagePlaceholder(size: size),
       ),
     );
   }
 }
 
-/// Full-width "hero" image atop the offer detail page. Same graceful
-/// degradation as [OfferThumbnail]; hidden entirely when the offer has no photo.
+/// Full-width "hero" image atop the offer detail page. Same source handling as
+/// [OfferThumbnail]; hidden entirely when the offer has no photo.
 class OfferHeroImage extends StatelessWidget {
   const OfferHeroImage({
     required this.url,
@@ -66,33 +63,61 @@ class OfferHeroImage extends StatelessWidget {
       borderRadius: BorderRadius.circular(14),
       child: AspectRatio(
         aspectRatio: 4 / 3,
-        child: Image.network(
-          url,
-          fit: BoxFit.cover,
-          semanticLabel: semanticLabel,
-          loadingBuilder: (context, child, progress) => progress == null
-              ? child
-              : const _OfferImagePlaceholder(loading: true),
-          errorBuilder: (context, _, _) => const _OfferImagePlaceholder(),
-        ),
+        child: _offerImage(url, semanticLabel: semanticLabel),
       ),
     );
   }
 }
 
-/// Neutral fill shown while a remote offer image loads or when it can't be
-/// fetched — a marketplace is useless if a broken image blocks the card.
-class _OfferImagePlaceholder extends StatelessWidget {
-  const _OfferImagePlaceholder({this.size, this.loading = false});
+/// Renders an offer image from either an inline `data:` URI (via memory) or a
+/// remote URL (via network), always cropping to fill and falling back to a
+/// neutral placeholder — a broken image must never block the card.
+Widget _offerImage(
+  String url, {
+  required String semanticLabel,
+  double? width,
+  double? height,
+}) {
+  final inline = decodeDataUri(url);
+  if (inline != null) {
+    return Image.memory(
+      inline,
+      width: width,
+      height: height,
+      fit: BoxFit.cover,
+      semanticLabel: semanticLabel,
+      errorBuilder: (context, _, _) =>
+          _OfferImagePlaceholder(width: width, height: height),
+    );
+  }
+  return Image.network(
+    url,
+    width: width,
+    height: height,
+    fit: BoxFit.cover,
+    semanticLabel: semanticLabel,
+    loadingBuilder: (context, child, progress) => progress == null
+        ? child
+        : _OfferImagePlaceholder(width: width, height: height, loading: true),
+    errorBuilder: (context, _, _) =>
+        _OfferImagePlaceholder(width: width, height: height),
+  );
+}
 
-  final double? size;
+/// Neutral fill shown while a remote offer image loads or when it can't be
+/// decoded/fetched — a marketplace is useless if a broken image blocks the card.
+class _OfferImagePlaceholder extends StatelessWidget {
+  const _OfferImagePlaceholder({this.width, this.height, this.loading = false});
+
+  final double? width;
+  final double? height;
   final bool loading;
 
   @override
   Widget build(BuildContext context) {
     return Container(
-      width: size,
-      height: size,
+      width: width,
+      height: height,
       color: seedPrimaryContainer.withValues(alpha: 0.4),
       alignment: Alignment.center,
       child: loading