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.

packages/commons_core/pubspec.yaml

@@ -21,11 +21,6 @@ dependencies:
   # signing, NIP-19, NIP-44, gift wrap — the vetted crypto for the social layer
   # transport. We keep the relay/websocket layer and our derivation on top.
   nostr: ^2.0.0
-  # Plain HTTP (BSD-3, AGPL-compatible) for the Blossom media server — offer
-  # photos are uploaded over HTTP, not the relay websocket. Client is injectable.
-  http: ^1.2.0
-  # Sync SHA-256 (BSD-3) for content-addressed Blossom blob ids.
-  crypto: ^3.0.3
 
 dev_dependencies:
   lints: ^6.0.0