feat(block2): messaging UI — private 1:1 chat (NIP-17)

- MessagesCubit: 1:1 conversation over MessageTransport. Filters the shared
  inbox to the peer; tags our own sends with our pubkey (NIP-17 wraps don't come
  back to the sender) so the UI can align bubbles; degrades gracefully offline.
- ChatScreen (mockup 10): message bubbles (mine right / peer left) + a composer;
  opens a session lazily, shows 'set up sharing' when there's no relay.
- Entry point: a 'Message' button on each offer card (hidden on your own
  listings) → /chat/:pubkey.
- Routing + i18n en/es/pt.

Tests: 5 cubit (send/receive/order/filter/offline) + chat offline + market, green.

apps/app_seeds/lib/i18n/en.i18n.json

@@ -342,6 +342,7 @@
     "searching": "Looking around your area…",
     "empty": "No seeds shared near you yet",
     "near": "Near you",
+    "contact": "Message",
     "configTitle": "Sharing setup",
     "setupIntro": "Sharing with people nearby is optional. To turn it on, set your rough area and join a community you trust — a seed network or collective that runs a shared meeting point for passing messages. No company in the middle.",
     "areaLabel": "Your area",
@@ -358,5 +359,12 @@
     "useLocation": "Use my approximate location",
     "locationFailed": "Couldn't get your location",
     "queued": "Saved — we'll share these when you're connected"
+  },
+  "chat": {
+    "title": "Chat",
+    "hint": "Write a message…",
+    "send": "Send",
+    "empty": "No messages yet — say hello",
+    "offline": "Set up sharing to send messages"
   }
 }