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/ui/market_screen.dart

@@ -1,6 +1,7 @@
 import 'package:commons_core/commons_core.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
+import 'package:go_router/go_router.dart';
 
 import '../data/variety_repository.dart';
 import '../i18n/strings.g.dart';
@@ -166,17 +167,27 @@ class _MarketScreenState extends State<MarketScreen> {
           ? const Center(child: CircularProgressIndicator())
           : BlocProvider.value(
               value: cubit,
-              child: MarketBody(onConfigure: _openConfig),
+              child: MarketBody(
+                onConfigure: _openConfig,
+                selfPubkey: widget.social.publicKeyHex,
+              ),
             ),
     );
   }
 }
 
 class MarketBody extends StatelessWidget {
-  const MarketBody({required this.onConfigure, super.key});
+  const MarketBody({
+    required this.onConfigure,
+    this.selfPubkey,
+    super.key,
+  });
 
   final VoidCallback onConfigure;
 
+  /// This user's own pubkey, so we don't offer to message our own listings.
+  final String? selfPubkey;
+
   @override
   Widget build(BuildContext context) {
     final t = context.t;
@@ -215,7 +226,15 @@ class MarketBody extends StatelessWidget {
           padding: const EdgeInsets.all(16),
           itemCount: state.offers.length,
           separatorBuilder: (_, _) => const SizedBox(height: 12),
-          itemBuilder: (context, i) => _OfferCard(offer: state.offers[i]),
+          itemBuilder: (context, i) {
+            final o = state.offers[i];
+            return _OfferCard(
+              offer: o,
+              onContact: o.authorPubkeyHex == selfPubkey
+                  ? null
+                  : () => context.go('/chat/${o.authorPubkeyHex}'),
+            );
+          },
         );
       },
     );
@@ -225,10 +244,13 @@ class MarketBody extends StatelessWidget {
 /// One discovered offer. Human words for the reciprocity mode; coarse "near you"
 /// instead of any precise location; price only when it's a sale.
 class _OfferCard extends StatelessWidget {
-  const _OfferCard({required this.offer});
+  const _OfferCard({required this.offer, this.onContact});
 
   final Offer offer;
 
+  /// Opens a private chat with the author; null for our own listings.
+  final VoidCallback? onContact;
+
   @override
   Widget build(BuildContext context) {
     final t = context.t;
@@ -263,8 +285,8 @@ class _OfferCard extends StatelessWidget {
               const Icon(Icons.place_outlined, size: 15, color: seedMuted),
               const SizedBox(width: 4),
               Text(t.market.near, style: const TextStyle(color: seedMuted, fontSize: 13)),
-              if (offer.type == OfferType.sale && offer.priceAmount != null) ...[
-                const Spacer(),
+              const Spacer(),
+              if (offer.type == OfferType.sale && offer.priceAmount != null)
                 Text(
                   '${offer.priceAmount} ${offer.priceCurrency ?? ''}'.trim(),
                   style: const TextStyle(
@@ -272,7 +294,12 @@ class _OfferCard extends StatelessWidget {
                     fontWeight: FontWeight.w600,
                   ),
                 ),
-              ],
+              if (onContact != null)
+                TextButton.icon(
+                  onPressed: onContact,
+                  icon: const Icon(Icons.chat_bubble_outline, size: 16),
+                  label: Text(t.market.contact),
+                ),
             ],
           ),
         ],