fix(market): search your zone by a coarse prefix, not the exact cell

Discovery searched the full 5-char geohash (~2.4 km) with an exact Nostr tag
match, so two devices in adjacent cells never found each other even though
publish emits the full NIP-52 prefix ladder. Search now coarsens the area to a
configurable precision (SocialSettings.searchPrecision, default 4 ~20 km) via
searchPrefix(), with a human-worded 'how far' selector (Very close / Around
here / My region -> 5/4/3) in the market setup sheet.

Also: surface a clear 'couldn't reach the servers' message when no relay
accepts a share (instead of 'shared 0'), and add wss://relay.comunes.org as the
first default relay (reliable community home; public relays are backup).

Tests: settings precision (default/clamp/roundtrip), searchPrefix helper,
neighbour-cell discovery regression (offers_cubit + commons_core nostr
transports), and the range-selector widget test.

apps/app_seeds/test/services/discovery_area_test.dart

@@ -0,0 +1,22 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/services/discovery_area.dart';
+
+void main() {
+  test('coarsens the area to the search precision', () {
+    expect(searchPrefix('sp3e9', 4), 'sp3e');
+    expect(searchPrefix('sp3e9', 3), 'sp3');
+    expect(searchPrefix('sp3e9', 5), 'sp3e9');
+  });
+
+  test('never asks for more than the area actually has', () {
+    expect(searchPrefix('sp', 4), 'sp');
+    expect(searchPrefix('', 4), '');
+  });
+
+  test('two neighbours in adjacent 5-char cells share a 4-char prefix', () {
+    // The crux of the bug: sp3e9 and sp3e8 are different ±2.4 km cells, but
+    // searching at precision 4 collapses both to 'sp3e', so they find each
+    // other. Publishing emits the ladder down to 'sp3e', so this matches.
+    expect(searchPrefix('sp3e9', 4), searchPrefix('sp3e8', 4));
+  });
+}

apps/app_seeds/test/services/social_settings_test.dart

@@ -47,4 +47,28 @@ void main() {
     await settings.setAreaGeohash('sp3e9');
     expect(await settings.isConfigured, isTrue);
   });
+
+  test('search precision defaults to 4 (a wide "around here")', () async {
+    expect(await settings.searchPrecision(), 4);
+  });
+
+  test('stores and reads back the search precision', () async {
+    await settings.setSearchPrecision(3);
+    expect(await settings.searchPrecision(), 3);
+    await settings.setSearchPrecision(5);
+    expect(await settings.searchPrecision(), 5);
+  });
+
+  test('search precision is clamped to the 3–5 range', () async {
+    await settings.setSearchPrecision(1);
+    expect(await settings.searchPrecision(), 3);
+    await settings.setSearchPrecision(9);
+    expect(await settings.searchPrecision(), 5);
+  });
+
+  test('a corrupt stored precision falls back to the default', () async {
+    final store = FakeSecretStore();
+    await store.write('tane.social.search_precision', 'oops');
+    expect(await SocialSettings(store).searchPrecision(), 4);
+  });
 }

apps/app_seeds/test/state/offers_cubit_test.dart

@@ -4,6 +4,7 @@ import 'package:commons_core/commons_core.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:tane/data/variety_repository.dart';
 import 'package:tane/db/enums.dart';
+import 'package:tane/services/discovery_area.dart';
 import 'package:tane/services/offer_mapper.dart';
 import 'package:tane/services/offer_outbox.dart';
 import 'package:tane/state/offers_cubit.dart';
@@ -152,6 +153,31 @@ void main() {
       await cubit.close();
     });
 
+    test('a neighbour in an adjacent cell is found via the coarsened prefix',
+        () async {
+      // Regression: A publishes at 'sp3e9', B lives one ±2.4 km cell over at
+      // 'sp3e8'. Searching B's full 5-char area misses it; searching the
+      // precision-4 prefix ('sp3e') finds it, because publish emits the ladder.
+      final transport = FakeOfferTransport();
+      await transport.publish(OfferMapper.fromSharedLot(
+        lotId: 'lot-1',
+        authorPubkeyHex: 'ab' * 32,
+        summary: 'Tomate rosa',
+        sharing: OfferStatus.shared,
+        areaGeohash: 'sp3e9',
+      ));
+      final cubit = OffersCubit(transport);
+
+      await cubit.discover(searchPrefix('sp3e8', 4)); // 'sp3e'
+      await pumpEventQueue();
+      expect(cubit.state.offers, hasLength(1));
+
+      await cubit.discover('sp3e8'); // full precision → the old, broken behaviour
+      await pumpEventQueue();
+      expect(cubit.state.offers, isEmpty);
+      await cubit.close();
+    });
+
     test('a different area finds nothing', () async {
       final transport = FakeOfferTransport();
       await transport.publish(OfferMapper.fromSharedLot(

apps/app_seeds/test/ui/market_screen_test.dart

@@ -91,6 +91,28 @@ void main() {
     expect(find.text('Retry'), findsOneWidget); // can't-reach state offers retry
   });
 
+  testWidgets('the range selector persists the chosen search precision',
+      (tester) async {
+    final social = await SocialService.fromRootSeedHex('00' * 32);
+    final settings = SocialSettings(InMemorySecretStore());
+    await settings.setRelayUrls(const []); // offline: don't hit the network
+
+    await tester.pumpWidget(_wrapMarket(social, settings));
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.byKey(const Key('market.config')));
+    await tester.pumpAndSettle();
+
+    // Defaults to the wide "Around here" (precision 4); narrow to "My region".
+    expect(await settings.searchPrecision(), 4);
+    await tester.tap(find.text('My region'));
+    await tester.pumpAndSettle();
+    await tester.tap(find.byKey(const Key('market.save')));
+    await tester.pumpAndSettle();
+
+    expect(await settings.searchPrecision(), 3);
+  });
+
   testWidgets('"use my location" fills the area with a coarse geohash',
       (tester) async {
     final social = await SocialService.fromRootSeedHex('00' * 32);