feat(market): host and publish the cover photo with each offer

Wire the Blossom MediaTransport into publishing: shareable lots carry
their varietyId, publishLots uploads the lot's cover photo and puts the
returned URL on the offer (best-effort — a hosting failure still publishes
the offer, just without a photo). Add a configurable media server (Comunes
default, empty to opt out) to settings and the market advanced config.

packages/commons_core/test/social/nostr_transports_test.dart

@@ -35,6 +35,7 @@ void main() {
         type: OfferType.gift,
         category: 'Solanaceae',
         approxGeohash: 'sp3e9xyz',
+        imageUrl: 'https://media.test/tomate.jpg',
       ));
       expect(published.accepted, isTrue, reason: published.message);
 
@@ -43,6 +44,7 @@ void main() {
       expect(found, hasLength(1));
       expect(found.single.summary, 'Tomate rosa de Barbastro');
       expect(found.single.authorPubkeyHex, alice.publicKeyHex);
+      expect(found.single.imageUrl, 'https://media.test/tomate.jpg');
 
       await aliceT.close();
       await bobT.close();