feat(market): show offer photos in the market list and detail

Render offer.imageUrl (already carried by NIP-99) as a thumbnail on each
market card and a hero image on the offer detail page. Remote images
degrade gracefully: a spinner while loading, a neutral placeholder when
the media server is unreachable. Publishing the photo comes next.

apps/app_seeds/lib/ui/market_widgets.dart

@@ -14,6 +14,98 @@ String offerTypeLabel(Translations t, OfferType type) => switch (type) {
       OfferType.lend => t.share.exchange,
     };
 
+/// 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.
+class OfferThumbnail extends StatelessWidget {
+  const OfferThumbnail({
+    required this.url,
+    required this.semanticLabel,
+    this.size = 56,
+    super.key,
+  });
+
+  final String url;
+  final String semanticLabel;
+  final double size;
+
+  @override
+  Widget build(BuildContext context) {
+    return ClipRRect(
+      borderRadius: BorderRadius.circular(10),
+      child: Image.network(
+        url,
+        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.
+class OfferHeroImage extends StatelessWidget {
+  const OfferHeroImage({
+    required this.url,
+    required this.semanticLabel,
+    super.key,
+  });
+
+  final String url;
+  final String semanticLabel;
+
+  @override
+  Widget build(BuildContext context) {
+    return ClipRRect(
+      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(),
+        ),
+      ),
+    );
+  }
+}
+
+/// 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});
+
+  final double? size;
+  final bool loading;
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      width: size,
+      height: size,
+      color: seedPrimaryContainer.withValues(alpha: 0.4),
+      alignment: Alignment.center,
+      child: loading
+          ? const SizedBox(
+              width: 20,
+              height: 20,
+              child: CircularProgressIndicator(strokeWidth: 2),
+            )
+          : const Icon(Icons.image_not_supported_outlined, color: seedMuted),
+    );
+  }
+}
+
 /// The small pill showing an offer's reciprocity mode (give/swap/sell/wanted).
 class OfferTypeChip extends StatelessWidget {
   const OfferTypeChip({required this.label, super.key});