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/i18n/en.i18n.json

@@ -366,5 +366,11 @@
     "send": "Send",
     "empty": "No messages yet — say hello",
     "offline": "Set up sharing to send messages"
+  },
+  "trust": {
+    "none": "No one vouches for them yet",
+    "count": "Vouched for by {n}",
+    "vouch": "I know this person",
+    "vouched": "You vouch for them"
   }
 }