feat(block2): trust UI — vouch for a person in chat (web of trust)

- TrustCubit: reads how many vouch for the peer + whether YOU do; toggleVouch
  certifies/revokes over TrustTransport; never vouches for self; offline-safe.
  Kept simple — the full known-member rule (threshold+distance) and its
  bootstrap set are still-open policy (network-trust §2); a live count + your
  vouch is the honest first step.
- Chat screen now opens ONE session carrying both messaging AND trust (the
  shared-connection shape), and shows a slim vouch banner: 'Vouched for by N' +
  an 'I know this person' / 'You vouch for them' toggle.
- i18n en/es/pt (human words: 'I know this person', not 'certify').

Tests: 4 trust cubit + messaging + chat, 10 green; analyzer clean.

apps/app_seeds/test/state/trust_cubit_test.dart

@@ -0,0 +1,82 @@
+import 'package:commons_core/commons_core.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/state/trust_cubit.dart';
+
+/// In-memory [TrustTransport]: certify/revoke act as this device's identity.
+class FakeTrustTransport implements TrustTransport {
+  FakeTrustTransport(this.selfId);
+  final String selfId;
+  final Map<String, Set<String>> _certs = {};
+
+  @override
+  Future<Set<String>> certifiersOf(String subjectPubkey) async =>
+      _certs[subjectPubkey] ?? <String>{};
+
+  @override
+  Future<void> certify({
+    required String subjectPubkey,
+    Duration validity = const Duration(days: 365),
+    String note = '',
+  }) async =>
+      (_certs[subjectPubkey] ??= {}).add(selfId);
+
+  @override
+  Future<void> revoke({required String subjectPubkey}) async =>
+      _certs[subjectPubkey]?.remove(selfId);
+
+  @override
+  Future<List<Certification>> allCertifications() async => const [];
+
+  @override
+  Future<void> close() async {}
+}
+
+void main() {
+  const me = 'me';
+  const peer = 'peer';
+
+  test('loads certifier count and whether I vouch', () async {
+    final transport = FakeTrustTransport(me);
+    await transport.certify(subjectPubkey: peer); // I already vouch
+    final cubit = TrustCubit(transport, peerPubkey: peer, selfPubkey: me);
+    await cubit.load();
+    expect(cubit.state.certifierCount, 1);
+    expect(cubit.state.iVouch, isTrue);
+    expect(cubit.state.loading, isFalse);
+    await cubit.close();
+  });
+
+  test('toggleVouch certifies then revokes', () async {
+    final cubit =
+        TrustCubit(FakeTrustTransport(me), peerPubkey: peer, selfPubkey: me);
+    await cubit.load();
+    expect(cubit.state.iVouch, isFalse);
+
+    await cubit.toggleVouch();
+    expect(cubit.state.iVouch, isTrue);
+    expect(cubit.state.certifierCount, 1);
+
+    await cubit.toggleVouch();
+    expect(cubit.state.iVouch, isFalse);
+    expect(cubit.state.certifierCount, 0);
+    await cubit.close();
+  });
+
+  test('never vouches for self', () async {
+    final cubit =
+        TrustCubit(FakeTrustTransport(me), peerPubkey: me, selfPubkey: me);
+    await cubit.load();
+    await cubit.toggleVouch();
+    expect(cubit.state.iVouch, isFalse);
+    await cubit.close();
+  });
+
+  test('offline (no transport) never throws', () async {
+    final cubit = TrustCubit(null, peerPubkey: peer, selfPubkey: me);
+    expect(cubit.isOnline, isFalse);
+    await cubit.load();
+    await cubit.toggleVouch();
+    expect(cubit.state.loading, isFalse);
+    await cubit.close();
+  });
+}