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/lib/ui/chat_screen.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:flutter/material.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
 
@@ -5,6 +7,7 @@ import '../i18n/strings.g.dart';
 import '../services/social_service.dart';
 import '../services/social_settings.dart';
 import '../state/messages_cubit.dart';
+import '../state/trust_cubit.dart';
 import 'theme.dart';
 
 /// A 1:1 encrypted chat with one peer (redesign screen 10). Private and
@@ -27,7 +30,9 @@ class ChatScreen extends StatefulWidget {
 }
 
 class _ChatScreenState extends State<ChatScreen> {
-  MessagesCubit? _cubit;
+  SocialSession? _session;
+  MessagesCubit? _messages;
+  TrustCubit? _trust;
   bool _loading = true;
   final _input = TextEditingController();
 
@@ -38,24 +43,38 @@ class _ChatScreenState extends State<ChatScreen> {
   }
 
   Future<void> _init() async {
-    final cubit = await createMessagesCubit(
-      widget.social,
-      widget.settings,
-      peerPubkey: widget.peerPubkey,
-    );
+    // 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;
+      }
+    }
     if (!mounted) {
-      await cubit.close();
+      await session?.close();
       return;
     }
-    cubit.start();
+    final self = widget.social.publicKeyHex;
+    final messages = MessagesCubit(session?.messages,
+        peerPubkey: widget.peerPubkey, selfPubkey: self)
+      ..start();
+    final trust = TrustCubit(session?.trust,
+        peerPubkey: widget.peerPubkey, selfPubkey: self);
+    unawaited(trust.load());
     setState(() {
-      _cubit = cubit;
+      _session = session;
+      _messages = messages;
+      _trust = trust;
       _loading = false;
     });
   }
 
   Future<void> _send() async {
-    final cubit = _cubit;
+    final cubit = _messages;
     if (cubit == null || _input.text.trim().isEmpty) return;
     final text = _input.text;
     _input.clear();
@@ -64,7 +83,9 @@ class _ChatScreenState extends State<ChatScreen> {
 
   @override
   void dispose() {
-    _cubit?.close();
+    _messages?.close();
+    _trust?.close();
+    _session?.close();
     _input.dispose();
     super.dispose();
   }
@@ -72,13 +93,17 @@ class _ChatScreenState extends State<ChatScreen> {
   @override
   Widget build(BuildContext context) {
     final t = context.t;
-    final cubit = _cubit;
+    final messages = _messages;
+    final trust = _trust;
     return Scaffold(
       appBar: AppBar(title: Text(t.chat.title)),
-      body: _loading || cubit == null
+      body: _loading || messages == null || trust == null
           ? const Center(child: CircularProgressIndicator())
-          : BlocProvider.value(
-              value: cubit,
+          : MultiBlocProvider(
+              providers: [
+                BlocProvider.value(value: messages),
+                BlocProvider.value(value: trust),
+              ],
               child: _ChatBody(controller: _input, onSend: _send),
             ),
     );
@@ -109,6 +134,7 @@ class _ChatBody extends StatelessWidget {
     }
     return Column(
       children: [
+        const _TrustBanner(),
         Expanded(
           child: BlocBuilder<MessagesCubit, ChatState>(
             builder: (context, state) {
@@ -135,6 +161,47 @@ class _ChatBody extends StatelessWidget {
   }
 }
 
+/// A slim "web of trust" strip: how many vouch for this peer, and a toggle to
+/// add/remove your own vouch ("I know this person").
+class _TrustBanner extends StatelessWidget {
+  const _TrustBanner();
+
+  @override
+  Widget build(BuildContext context) {
+    final t = context.t;
+    return BlocBuilder<TrustCubit, TrustState>(
+      builder: (context, state) {
+        final cubit = context.read<TrustCubit>();
+        if (!cubit.isOnline || state.loading) return const SizedBox.shrink();
+        return Container(
+          width: double.infinity,
+          color: seedPrimaryContainer.withValues(alpha: 0.5),
+          padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 8),
+          child: Row(
+            children: [
+              const Icon(Icons.verified_user_outlined, size: 18, color: seedGreen),
+              const SizedBox(width: 8),
+              Expanded(
+                child: Text(
+                  state.certifierCount == 0
+                      ? t.trust.none
+                      : t.trust.count(n: state.certifierCount),
+                  style: const TextStyle(color: seedOnSurface, fontSize: 13),
+                ),
+              ),
+              TextButton(
+                key: const Key('chat.vouch'),
+                onPressed: state.busy ? null : cubit.toggleVouch,
+                child: Text(state.iVouch ? t.trust.vouched : t.trust.vouch),
+              ),
+            ],
+          ),
+        );
+      },
+    );
+  }
+}
+
 class _Bubble extends StatelessWidget {
   const _Bubble({required this.text, required this.mine});