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);
+  });
 }