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/strings.g.dart

@@ -4,9 +4,9 @@
 /// To regenerate, run: `dart run slang`
 ///
 /// Locales: 3
-/// Strings: 956 (318 per locale)
+/// Strings: 974 (324 per locale)
 ///
-/// Built on 2026-07-10 at 08:49 UTC
+/// Built on 2026-07-10 at 08:58 UTC
 
 // coverage:ignore-file
 // ignore_for_file: type=lint, unused_import