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/ui/market_screen.dart

@@ -603,7 +603,6 @@ class _ConfigSheet extends StatefulWidget {
 class _ConfigSheetState extends State<_ConfigSheet> {
   final _area = TextEditingController();
   final _servers = TextEditingController();
-  final _mediaServer = TextEditingController();
   bool _loading = true;
   bool _locationBusy = false;
   String? _locationError;
@@ -618,7 +617,6 @@ class _ConfigSheetState extends State<_ConfigSheet> {
   Future<void> _load() async {
     _area.text = await widget.settings.areaGeohash();
     _servers.text = (await widget.settings.relayUrls()).join('\n');
-    _mediaServer.text = await widget.settings.mediaServerUrl();
     _precision = await widget.settings.searchPrecision();
     if (mounted) setState(() => _loading = false);
   }
@@ -626,7 +624,6 @@ class _ConfigSheetState extends State<_ConfigSheet> {
   Future<void> _save() async {
     await widget.settings.setAreaGeohash(_area.text);
     await widget.settings.setRelayUrls(_servers.text.split('\n'));
-    await widget.settings.setMediaServerUrl(_mediaServer.text);
     await widget.settings.setSearchPrecision(_precision);
     if (mounted) Navigator.of(context).pop(true);
   }
@@ -656,7 +653,6 @@ class _ConfigSheetState extends State<_ConfigSheet> {
   void dispose() {
     _area.dispose();
     _servers.dispose();
-    _mediaServer.dispose();
     super.dispose();
   }
 
@@ -796,16 +792,6 @@ class _ConfigSheetState extends State<_ConfigSheet> {
                             helperMaxLines: 2,
                           ),
                         ),
-                        const SizedBox(height: 14),
-                        TextField(
-                          key: const Key('market.mediaServer'),
-                          controller: _mediaServer,
-                          decoration: InputDecoration(
-                            labelText: t.market.mediaServerLabel,
-                            helperText: t.market.mediaServerHelp,
-                            helperMaxLines: 2,
-                          ),
-                        ),
                       ],
                     ),
                   ),