refactor(social): one shared relay connection per identity

Every feature (offers, messaging, trust, profile, the inbox listener) used
to open its OWN RelayPool via social.openSession — several sockets to the
same relays, more battery, and reconnection logic living only in the inbox
listener.

Add SocialConnection: ONE shared session per identity, lazily connected and
reused by all. It watches connectivity — dropping the session when the
network goes and reconnecting when it returns, announcing each change on a
sessions stream so the inbox listener re-subscribes automatically. Callers
get the session via connection.session() and never close it (the connection
owns its lifecycle); recreated/disposed on an identity switch.

- InboxService now consumes the shared connection (subscribes on its
  sessions stream) instead of owning its own socket + connectivity code.
- createOffersCubit + the chat/market/profile screens use the shared
  connection; dead createMessagesCubit/createTrustCubit factories removed.
- DI registers SocialConnection per identity; Bootstrap starts it (after
  the inbox subscribes); switchSocialAccount disposes + recreates it.

Tests: SocialConnection lifecycle (reuse, concurrent connect, offline drop +
reconnect, unreachable retry) with a fake opener + online stream; inbox test
updated. nostr added as a dev_dependency for the channel fake.

apps/app_seeds/lib/ui/market_screen.dart

@@ -8,6 +8,7 @@ import '../i18n/strings.g.dart';
 import '../services/coarse_location.dart';
 import '../services/discovery_area.dart';
 import '../services/offer_outbox.dart';
+import '../services/social_connection.dart';
 import '../services/social_service.dart';
 import '../services/social_settings.dart';
 import '../state/offers_cubit.dart';
@@ -21,6 +22,7 @@ class MarketScreen extends StatefulWidget {
   const MarketScreen({
     required this.social,
     required this.settings,
+    required this.connection,
     this.location,
     this.outbox,
     super.key,
@@ -29,6 +31,9 @@ class MarketScreen extends StatefulWidget {
   final SocialService social;
   final SocialSettings settings;
 
+  /// The shared relay connection (one per identity), reused for discovery.
+  final SocialConnection connection;
+
   /// Optional device-location source for the "use my location" shortcut; when
   /// null (tests, or a platform without it) the shortcut is hidden.
   final CoarseLocationProvider? location;
@@ -58,11 +63,8 @@ class _MarketScreenState extends State<MarketScreen> {
     final repo =
         widget.outbox != null ? context.read<VarietyRepository>() : null;
     setState(() => _loading = true);
-    final cubit = await createOffersCubit(
-      widget.social,
-      widget.settings,
-      repository: repo,
-    );
+    final cubit =
+        await createOffersCubit(widget.connection, repository: repo);
     final area = await widget.settings.areaGeohash();
     if (!mounted) {
       await cubit.close();