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/bootstrap.dart

@@ -19,6 +19,7 @@ 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';
@@ -53,6 +54,9 @@ class _BootstrapState extends State<Bootstrap> {
     if (savedLocale != null) LocaleSettings.setLocaleSync(savedLocale);
 
     final onboarding = getIt<OnboardingStore>();
+    final connection = getIt.isRegistered<SocialConnection>()
+        ? getIt<SocialConnection>()
+        : null;
     final inbox =
         getIt.isRegistered<InboxService>() ? getIt<InboxService>() : null;
     final notifications = getIt.isRegistered<NotificationService>()
@@ -61,9 +65,10 @@ class _BootstrapState extends State<Bootstrap> {
     // Ask for notification permission and set up the OS channel (no-op on
     // unsupported platforms). Done before the inbox starts listening.
     if (notifications != null) await notifications.initialize();
-    // Listen for incoming private messages app-wide (foreground) so they arrive
-    // and fill the inbox even before their chat is opened.
-    if (inbox != null) unawaited(inbox.start());
+    // Subscribe the inbox listener BEFORE the shared connection starts
+    // connecting, so the first session is caught; then bring the connection up.
+    inbox?.start();
+    connection?.start();
 
     return TaneApp(
       repository: getIt<VarietyRepository>(),
@@ -72,6 +77,7 @@ class _BootstrapState extends State<Bootstrap> {
       social:
           getIt.isRegistered<SocialService>() ? getIt<SocialService>() : null,
       socialSettings: getIt<SocialSettings>(),
+      connection: connection,
       location: const GeolocatorCoarseLocation(),
       outbox: getIt<OfferOutbox>(),
       messageStore: getIt<MessageStore>(),