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/i18n/strings_en.g.dart

@@ -1285,12 +1285,6 @@ class Translations$market$en {
 
 	/// en: 'Photo'
 	String get photo => 'Photo';
-
-	/// en: 'Photo server'
-	String get mediaServerLabel => 'Photo server';
-
-	/// en: 'Where offer photos are hosted so others can see them. Leave empty to share without photos.'
-	String get mediaServerHelp => 'Where offer photos are hosted so others can see them. Leave empty to share without photos.';
 }
 
 // Path: profile
@@ -2230,8 +2224,6 @@ extension on Translations {
 			'market.copyId' => 'Copy code',
 			'market.idCopied' => 'Code copied',
 			'market.photo' => 'Photo',
-			'market.mediaServerLabel' => 'Photo server',
-			'market.mediaServerHelp' => 'Where offer photos are hosted so others can see them. Leave empty to share without photos.',
 			'profile.title' => 'Your profile',
 			'profile.name' => 'Display name',
 			'profile.nameHint' => 'How others see you',