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/en.i18n.json

@@ -486,9 +486,7 @@
     "noProfile": "This person hasn't shared a profile yet",
     "copyId": "Copy code",
     "idCopied": "Code copied",
-    "photo": "Photo",
-    "mediaServerLabel": "Photo server",
-    "mediaServerHelp": "Where offer photos are hosted so others can see them. Leave empty to share without photos."
+    "photo": "Photo"
   },
   "profile": {
     "title": "Your profile",