feat(block2): social settings + offers cubit factory (area, relays)

Persistence + wiring the offers screen needs, per the product decisions:
- SocialSettings (keystore-backed, no plaintext): coarse area geohash +
  configurable relay URLs, both empty by default (no bundled public relays —
  don't leak metadata to third parties). Registered in DI.
- createOffersCubit(): opens a SocialSession on the first configured relay when
  online, else a null-transport cubit that degrades gracefully. OffersCubit now
  closes the owning session on dispose.

4 settings tests green; offers/social suites still green.

apps/app_seeds/test/services/social_settings_test.dart

@@ -0,0 +1,45 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/security/secret_store.dart';
+import 'package:tane/services/social_settings.dart';
+
+/// In-memory [SecretStore] for tests.
+class FakeSecretStore implements SecretStore {
+  final Map<String, String> _data = {};
+  @override
+  Future<String?> read(String key) async => _data[key];
+  @override
+  Future<void> write(String key, String value) async => _data[key] = value;
+}
+
+void main() {
+  late SocialSettings settings;
+  setUp(() => settings = SocialSettings(FakeSecretStore()));
+
+  test('defaults: empty area, no relays, not configured', () async {
+    expect(await settings.areaGeohash(), '');
+    expect(await settings.relayUrls(), isEmpty);
+    expect(await settings.isConfigured, isFalse);
+  });
+
+  test('stores and normalises the area geohash (trim + lowercase)', () async {
+    await settings.setAreaGeohash('  SP3E9  ');
+    expect(await settings.areaGeohash(), 'sp3e9');
+  });
+
+  test('stores relay urls, dropping blanks', () async {
+    await settings.setRelayUrls([
+      'wss://relay.comunes.org',
+      '  ',
+      'wss://seeds.example.org',
+    ]);
+    expect(await settings.relayUrls(),
+        ['wss://relay.comunes.org', 'wss://seeds.example.org']);
+  });
+
+  test('is configured only with both an area and at least one relay', () async {
+    await settings.setAreaGeohash('sp3e9');
+    expect(await settings.isConfigured, isFalse);
+    await settings.setRelayUrls(['wss://relay.comunes.org']);
+    expect(await settings.isConfigured, isTrue);
+  });
+}