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/state/messages_cubit.dart

@@ -5,8 +5,6 @@ import 'package:equatable/equatable.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
 
 import '../services/message_store.dart';
-import '../services/social_service.dart';
-import '../services/social_settings.dart';
 
 /// A 1:1 conversation with one peer. Holds the running message list plus send
 /// status. Transport-agnostic — depends on [MessageTransport], not the relay.
@@ -134,29 +132,3 @@ class MessagesCubit extends Cubit<ChatState> {
     return super.close();
   }
 }
-
-/// Opens a [MessagesCubit] wired to the social layer for a chat with
-/// [peerPubkey], or an offline one that degrades gracefully.
-Future<MessagesCubit> createMessagesCubit(
-  SocialService social,
-  SocialSettings settings, {
-  required String peerPubkey,
-}) async {
-  final relays = await settings.relayUrls();
-  if (relays.isEmpty) {
-    return MessagesCubit(null,
-        peerPubkey: peerPubkey, selfPubkey: social.publicKeyHex);
-  }
-  try {
-    final session = await social.openSession(relays);
-    return MessagesCubit(
-      session.messages,
-      peerPubkey: peerPubkey,
-      selfPubkey: social.publicKeyHex,
-      onDispose: session.close,
-    );
-  } catch (_) {
-    return MessagesCubit(null,
-        peerPubkey: peerPubkey, selfPubkey: social.publicKeyHex);
-  }
-}

apps/app_seeds/lib/state/offers_cubit.dart

@@ -9,8 +9,7 @@ import '../data/variety_repository.dart';
 import '../services/offer_mapper.dart';
 import '../services/offer_outbox.dart';
 import '../services/offer_thumbnail.dart';
-import '../services/social_service.dart';
-import '../services/social_settings.dart';
+import '../services/social_connection.dart';
 
 /// State of the offer discovery/publish screen. Transport-agnostic — it holds
 /// only what the UI shows, never a relay handle.
@@ -323,27 +322,20 @@ class OffersCubit extends Cubit<OffersState> {
   }
 }
 
-/// Opens an [OffersCubit] wired to the social layer, or an offline one that
-/// degrades gracefully. Local-first: when no relay is configured (or connecting
-/// fails), the cubit is created with a null transport and the screen still opens.
+/// Opens an [OffersCubit] over the SHARED [SocialConnection], or an offline one
+/// that degrades gracefully. Local-first: when the connection isn't up (no relay
+/// configured / unreachable), the cubit gets a null transport and the screen
+/// still opens. Does NOT close the session — the connection owns it.
 Future<OffersCubit> createOffersCubit(
-  SocialService social,
-  SocialSettings settings, {
+  SocialConnection connection, {
   VarietyRepository? repository,
 }) async {
-  final relays = await settings.relayUrls();
-  if (relays.isEmpty) return OffersCubit(null);
-  try {
-    final session = await social.openSession(relays);
-    return OffersCubit(
-      session.offers,
-      coverPhoto: repository?.coverPhotoFor,
-      thumbnail: offerThumbnailDataUri,
-      onDispose: session.close,
-    );
-  } catch (_) {
-    return OffersCubit(null); // offline / no relay reachable
-  }
+  final session = await connection.session();
+  return OffersCubit(
+    session?.offers,
+    coverPhoto: repository?.coverPhotoFor,
+    thumbnail: offerThumbnailDataUri,
+  );
 }
 
 /// Publishes any queued (offline-parked) lots now that we're online, then clears

apps/app_seeds/lib/state/trust_cubit.dart

@@ -2,9 +2,6 @@ import 'package:commons_core/commons_core.dart';
 import 'package:equatable/equatable.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
 
-import '../services/social_service.dart';
-import '../services/social_settings.dart';
-
 /// Where a peer stands, from strongest to weakest signal.
 enum TrustTier {
   /// Passes the full Duniter membership rule (sigQty certifications from members,
@@ -189,29 +186,3 @@ class TrustCubit extends Cubit<TrustState> {
     return super.close();
   }
 }
-
-/// Opens a [TrustCubit] wired to the social layer for [peerPubkey], or an
-/// offline one.
-Future<TrustCubit> createTrustCubit(
-  SocialService social,
-  SocialSettings settings, {
-  required String peerPubkey,
-}) async {
-  final relays = await settings.relayUrls();
-  if (relays.isEmpty) {
-    return TrustCubit(null,
-        peerPubkey: peerPubkey, selfPubkey: social.publicKeyHex);
-  }
-  try {
-    final session = await social.openSession(relays);
-    return TrustCubit(
-      session.trust,
-      peerPubkey: peerPubkey,
-      selfPubkey: social.publicKeyHex,
-      onDispose: session.close,
-    );
-  } catch (_) {
-    return TrustCubit(null,
-        peerPubkey: peerPubkey, selfPubkey: social.publicKeyHex);
-  }
-}