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/en.i18n.json

@@ -361,6 +361,10 @@
     "locationFailed": "Couldn't get your location — check that location is on and the permission is granted",
     "queued": "Saved — we'll share these when you're connected"
   },
+  "chatList": {
+    "title": "Messages",
+    "empty": "No conversations yet. Message someone from the market."
+  },
   "chat": {
     "title": "Chat",
     "hint": "Write a message…",