feat(block2): market screen — discover seeds near you (offers UI)

Wires the 'coming soon' market card to a real discovery screen (mockups 04/05):
- MarketScreen: opens an OfferTransport lazily; local-first, so it degrades to a
  human 'set up sharing' prompt when no relay/area is set or the net is down.
- Discovers offers by the saved coarse area, lists them with human reciprocity
  labels (gift/swap/for sale), 'near you' (never exact location), price only for
  sales. Config sheet to set area + community servers (no bundled public relays).
- Threaded via TaneApp(social, socialSettings) — null keeps the card 'coming
  soon' (so Block 1 tests pass unchanged); main wires it live.
- i18n en/es/pt (market.*), reuses share.* for type labels.

3 market widget tests + home tests green; app_seeds analyzes clean.

apps/app_seeds/test/ui/market_screen_test.dart

@@ -0,0 +1,123 @@
+import 'dart:async';
+
+import 'package:commons_core/commons_core.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_bloc/flutter_bloc.dart';
+import 'package:flutter_localizations/flutter_localizations.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/i18n/strings.g.dart';
+import 'package:tane/services/social_service.dart';
+import 'package:tane/services/social_settings.dart';
+import 'package:tane/state/offers_cubit.dart';
+import 'package:tane/ui/market_screen.dart';
+
+import '../support/test_support.dart';
+
+/// In-memory offer transport: discover matches by geohash prefix.
+class FakeOfferTransport implements OfferTransport {
+  final List<Offer> offers;
+  FakeOfferTransport(this.offers);
+
+  @override
+  Future<PublishResult> publish(Offer offer) async =>
+      PublishResult(accepted: true, transportRef: offer.id);
+
+  @override
+  Stream<Offer> discover(DiscoveryQuery query) {
+    final c = StreamController<Offer>();
+    for (final o in offers) {
+      if (o.approxGeohash.startsWith(query.geohashPrefix)) c.add(o);
+    }
+    return c.stream;
+  }
+
+  @override
+  Future<void> retract(String offerId) async {}
+  @override
+  Future<void> close() async {}
+}
+
+Widget _wrapBody(OffersCubit cubit) {
+  LocaleSettings.setLocaleSync(AppLocale.en);
+  return TranslationProvider(
+    child: MaterialApp(
+      locale: AppLocale.en.flutterLocale,
+      supportedLocales: AppLocaleUtils.supportedLocales,
+      localizationsDelegates: const [
+        GlobalMaterialLocalizations.delegate,
+        GlobalWidgetsLocalizations.delegate,
+        GlobalCupertinoLocalizations.delegate,
+      ],
+      home: Scaffold(
+        body: BlocProvider.value(
+          value: cubit,
+          child: MarketBody(onConfigure: () {}),
+        ),
+      ),
+    ),
+  );
+}
+
+Offer _offer(String id, String summary, OfferType type) => Offer(
+      id: id,
+      authorPubkeyHex: 'ab' * 32,
+      summary: summary,
+      type: type,
+      approxGeohash: 'sp3e9',
+    );
+
+void main() {
+  testWidgets('renders discovered offers with their type', (tester) async {
+    final cubit = OffersCubit(FakeOfferTransport([
+      _offer('1', 'Tomate rosa de Barbastro', OfferType.gift),
+      _offer('2', 'Judía del ganxet', OfferType.exchange),
+    ]));
+    addTearDown(cubit.close);
+
+    await tester.pumpWidget(_wrapBody(cubit));
+    await cubit.discover('sp3');
+    await tester.pumpAndSettle();
+
+    expect(find.text('Tomate rosa de Barbastro'), findsOneWidget);
+    expect(find.text('Judía del ganxet'), findsOneWidget);
+    expect(find.text('Near you'), findsWidgets);
+  });
+
+  testWidgets('offline (no transport) shows the "set up sharing" prompt',
+      (tester) async {
+    final cubit = OffersCubit(null);
+    addTearDown(cubit.close);
+
+    await tester.pumpWidget(_wrapBody(cubit));
+    await tester.pumpAndSettle();
+
+    expect(find.text("Sharing isn't set up yet"), findsOneWidget);
+    expect(find.text('Set up sharing'), findsOneWidget);
+  });
+
+  testWidgets('MarketScreen with no relays configured degrades to offline',
+      (tester) async {
+    final social = await SocialService.fromRootSeedHex('00' * 32);
+    final settings = SocialSettings(InMemorySecretStore());
+    LocaleSettings.setLocaleSync(AppLocale.en);
+
+    await tester.pumpWidget(
+      TranslationProvider(
+        child: MaterialApp(
+          locale: AppLocale.en.flutterLocale,
+          supportedLocales: AppLocaleUtils.supportedLocales,
+          localizationsDelegates: const [
+            GlobalMaterialLocalizations.delegate,
+            GlobalWidgetsLocalizations.delegate,
+            GlobalCupertinoLocalizations.delegate,
+          ],
+          home: MarketScreen(social: social, settings: settings),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    expect(find.text('Seeds near you'), findsOneWidget); // app bar title
+    expect(find.text('Set up sharing'), findsOneWidget); // offline prompt
+  });
+}