feat(block2): messages inbox + drawer 'Chat' link (was unreachable)

Chat was only reachable via an offer's 'Message' button, and the drawer 'Chat'
sat on 'coming soon' — so messaging/trust were effectively hidden.
- MessageStore keeps a conversation index (the keystore has no key enumeration)
  and exposes conversations() — peers newest-active-first with their last line.
- ChatListScreen: the messages inbox (tap a conversation → /chat/:pubkey).
- Drawer 'Chat' is now a live destination -> /messages (gated like Market).
- i18n en/es/pt. conversations() covered by a plain test (no widget hang).

Analyzer clean.

apps/app_seeds/lib/i18n/es.i18n.json

@@ -361,6 +361,10 @@
     "locationFailed": "No se pudo obtener tu ubicación — comprueba que la ubicación está activada y el permiso concedido",
     "queued": "Guardado — las compartiremos cuando tengas conexión"
   },
+  "chatList": {
+    "title": "Mensajes",
+    "empty": "Aún no hay conversaciones. Escribe a alguien desde el mercado."
+  },
   "chat": {
     "title": "Chat",
     "hint": "Escribe un mensaje…",