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);
-  }
-}