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/app.dart

@@ -18,6 +18,7 @@ import 'services/onboarding_store.dart';
 import 'services/profile_cache.dart';
 import 'services/profile_store.dart';
 import 'services/social_account_store.dart';
+import 'services/social_connection.dart';
 import 'services/social_service.dart';
 import 'services/social_settings.dart';
 import 'services/trust_referents.dart';
@@ -50,6 +51,7 @@ class TaneApp extends StatelessWidget {
     required this.onboarding,
     this.social,
     this.socialSettings,
+    this.connection,
     this.location,
     this.outbox,
     this.messageStore,
@@ -69,6 +71,7 @@ class TaneApp extends StatelessWidget {
           showIntro,
           social,
           socialSettings,
+          connection,
           location,
           outbox,
           messageStore,
@@ -94,6 +97,9 @@ class TaneApp extends StatelessWidget {
   final SocialService? social;
   final SocialSettings? socialSettings;
 
+  /// The shared relay connection (one per identity) reused by every feature.
+  final SocialConnection? connection;
+
   /// Optional device-location source for the market's "use my location".
   final CoarseLocationProvider? location;
 
@@ -135,6 +141,7 @@ class TaneApp extends StatelessWidget {
     bool showIntro,
     SocialService? social,
     SocialSettings? socialSettings,
+    SocialConnection? connection,
     CoarseLocationProvider? location,
     OfferOutbox? outbox,
     MessageStore? messageStore,
@@ -153,50 +160,49 @@ class TaneApp extends StatelessWidget {
           builder: (context, state) =>
               HomeScreen(marketEnabled: social != null),
         ),
-        if (social != null && socialSettings != null)
+        if (social != null && socialSettings != null && connection != null)
           GoRoute(
             path: '/market',
             builder: (context, state) => MarketScreen(
               social: social,
               settings: socialSettings,
+              connection: connection,
               location: location,
               outbox: outbox,
             ),
           ),
-        if (social != null && socialSettings != null)
+        if (social != null && connection != null)
           GoRoute(
             path: '/market/offer',
             builder: (context, state) {
               final offer = state.extra as Offer;
               return MarketOfferDetailScreen(
                 offer: offer,
-                social: social,
-                settings: socialSettings,
+                connection: connection,
                 mine: offer.authorPubkeyHex == social.publicKeyHex,
                 profileCache: profileCache,
               );
             },
           ),
-        if (social != null && socialSettings != null && messageStore != null)
+        if (messageStore != null)
           GoRoute(
             path: '/messages',
             builder: (context, state) => ChatListScreen(
               store: messageStore,
-              social: social,
-              settings: socialSettings,
+              connection: connection,
               profileCache: profileCache,
               inbox: inbox,
             ),
           ),
         if (social != null &&
-            socialSettings != null &&
+            connection != null &&
             profileStore != null &&
             socialAccounts != null)
           GoRoute(
             path: '/profile',
             builder: (context, state) => ProfileScreen(
               social: social,
-              settings: socialSettings,
+              connection: connection,
               profileStore: profileStore,
               accounts: socialAccounts,
               trustNetworkEnabled:
@@ -211,12 +217,12 @@ class TaneApp extends StatelessWidget {
               wotSettings: wotSettings,
             ),
           ),
-        if (social != null && socialSettings != null)
+        if (social != null && connection != null)
           GoRoute(
             path: '/chat/:pubkey',
             builder: (context, state) => ChatScreen(
               social: social,
-              settings: socialSettings,
+              connection: connection,
               peerPubkey: state.pathParameters['pubkey']!,
               messageStore: messageStore,
               profileCache: profileCache,