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/chat_screen.dart

@@ -10,8 +10,8 @@ import '../di/injector.dart';
 import '../i18n/strings.g.dart';
 import '../services/message_store.dart';
 import '../services/profile_cache.dart';
+import '../services/social_connection.dart';
 import '../services/social_service.dart';
-import '../services/social_settings.dart';
 import '../services/trust_referents.dart';
 import '../services/unread_service.dart';
 import '../services/wot_settings.dart';
@@ -25,7 +25,7 @@ import 'theme.dart';
 class ChatScreen extends StatefulWidget {
   const ChatScreen({
     required this.social,
-    required this.settings,
+    required this.connection,
     required this.peerPubkey,
     this.messageStore,
     this.profileCache,
@@ -35,7 +35,9 @@ class ChatScreen extends StatefulWidget {
   });
 
   final SocialService social;
-  final SocialSettings settings;
+
+  /// The shared relay connection (one per identity), carrying messaging + trust.
+  final SocialConnection connection;
   final String peerPubkey;
 
   /// Optional persistence for chat history (keystore-backed); null in tests.
@@ -53,7 +55,6 @@ class ChatScreen extends StatefulWidget {
 }
 
 class _ChatScreenState extends State<ChatScreen> {
-  SocialSession? _session;
   MessagesCubit? _messages;
   TrustCubit? _trust;
   bool _loading = true;
@@ -79,25 +80,14 @@ class _ChatScreenState extends State<ChatScreen> {
   }
 
   Future<void> _init() async {
-    // One session for this chat carries both messaging and trust (the shared-
-    // connection shape). Offline (no relays / unreachable) → null transports.
-    final relays = await widget.settings.relayUrls();
-    SocialSession? session;
-    if (relays.isNotEmpty) {
-      try {
-        session = await widget.social.openSession(relays);
-      } catch (_) {
-        session = null;
-      }
-    }
+    // The shared connection carries both messaging and trust. Offline (not
+    // connected / unreachable) → null transports and the screen degrades.
+    final session = await widget.connection.session();
     final cachedName = await widget.profileCache?.name(widget.peerPubkey);
     final referents = await widget.trustReferents?.all() ?? const <String>{};
     final wotParams =
         await widget.wotSettings?.params() ?? WotParams.duniter;
-    if (!mounted) {
-      await session?.close();
-      return;
-    }
+    if (!mounted) return; // shared session is owned by the connection, not us
     final self = widget.social.publicKeyHex;
     final messages = MessagesCubit(
       session?.messages,
@@ -114,7 +104,6 @@ class _ChatScreenState extends State<ChatScreen> {
     );
     unawaited(trust.load());
     setState(() {
-      _session = session;
       _messages = messages;
       _trust = trust;
       _peerName = cachedName;
@@ -184,7 +173,7 @@ class _ChatScreenState extends State<ChatScreen> {
     unawaited(_unread?.markRead(widget.peerPubkey));
     _messages?.close();
     _trust?.close();
-    _session?.close();
+    // The session belongs to the shared connection — don't close it here.
     _input.dispose();
     super.dispose();
   }