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/ui/app_drawer.dart

@@ -49,6 +49,12 @@ class AppDrawer extends StatelessWidget {
             _DrawerItem(
               icon: const Icon(Icons.chat_bubble),
               label: t.menu.chat,
+              onTap: marketEnabled
+                  ? () {
+                      Navigator.of(context).pop();
+                      context.go('/messages');
+                    }
+                  : null,
             ),
             _DrawerItem(
               icon: const Icon(Icons.favorite),