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/test/services/social_connection_test.dart

@@ -0,0 +1,119 @@
+import 'dart:async';
+
+import 'package:commons_core/commons_core.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:nostr/nostr.dart';
+import 'package:tane/services/social_connection.dart';
+import 'package:tane/services/social_service.dart';
+import 'package:tane/services/social_settings.dart';
+
+import '../support/test_support.dart';
+
+/// A no-op [NostrChannel] that only tracks whether it was closed — enough to
+/// build a [SocialSession] and assert the connection's lifecycle.
+class FakeChannel implements NostrChannel {
+  bool closed = false;
+
+  @override
+  String get privateKeyHex => '00' * 32;
+  @override
+  String get publicKeyHex => 'ab' * 32;
+  @override
+  Future<({bool accepted, String message})> publish(Event event) async =>
+      (accepted: true, message: '');
+  @override
+  Stream<Event> subscribe(Filter filter) => const Stream.empty();
+  @override
+  Future<List<Event>> reqOnce(Filter filter) async => const [];
+  @override
+  Future<void> close() async => closed = true;
+}
+
+void main() {
+  const seedHex =
+      '000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f';
+
+  late SocialService social;
+  late SocialSettings settings; // relayUrls() falls back to defaults (non-empty)
+
+  setUp(() async {
+    social = await SocialService.fromRootSeedHex(seedHex);
+    settings = SocialSettings(InMemorySecretStore());
+  });
+
+  SocialConnection make({
+    required List<FakeChannel> opened,
+    Stream<bool>? online,
+    bool Function()? fail,
+  }) =>
+      SocialConnection(
+        social: social,
+        settings: settings,
+        online: online,
+        open: (_) async {
+          if (fail?.call() ?? false) throw StateError('unreachable');
+          final ch = FakeChannel();
+          opened.add(ch);
+          return SocialSession(ch);
+        },
+      );
+
+  test('connects once and reuses the shared session', () async {
+    final opened = <FakeChannel>[];
+    final conn = make(opened: opened);
+    final a = await conn.session();
+    final b = await conn.session();
+    expect(a, isNotNull);
+    expect(identical(a, b), isTrue); // same shared instance
+    expect(opened, hasLength(1)); // only one connection opened
+    await conn.dispose();
+  });
+
+  test('concurrent callers share a single connect', () async {
+    final opened = <FakeChannel>[];
+    final conn = make(opened: opened);
+    final results = await Future.wait([conn.session(), conn.session()]);
+    expect(identical(results[0], results[1]), isTrue);
+    expect(opened, hasLength(1));
+    await conn.dispose();
+  });
+
+  test('drops when offline and reconnects when back online', () async {
+    final opened = <FakeChannel>[];
+    final online = StreamController<bool>.broadcast();
+    final conn = make(opened: opened, online: online.stream);
+    final emitted = <SocialSession?>[];
+    conn.sessions.listen(emitted.add);
+
+    conn.start(); // watch connectivity + initial connect
+    final first = await conn.session();
+    expect(first, isNotNull);
+    expect(opened, hasLength(1));
+
+    online.add(false); // network lost
+    await Future<void>.delayed(Duration.zero);
+    expect(conn.current, isNull);
+    expect(opened.first.closed, isTrue); // old session closed
+    expect(emitted.last, isNull); // announced the drop
+
+    online.add(true); // network back
+    await Future<void>.delayed(Duration.zero);
+    expect(conn.current, isNotNull);
+    expect(opened, hasLength(2)); // reconnected
+    expect(identical(emitted.last, conn.current), isTrue);
+
+    await conn.dispose();
+    await online.close();
+  });
+
+  test('returns null when the relay is unreachable, and retries later',
+      () async {
+    final opened = <FakeChannel>[];
+    var down = true;
+    final conn = make(opened: opened, fail: () => down);
+    expect(await conn.session(), isNull); // unreachable now
+    down = false;
+    expect(await conn.session(), isNotNull); // succeeds on retry
+    await conn.dispose();
+  });
+}