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/lib/di/injector.dart

@@ -25,6 +25,7 @@ import '../services/onboarding_store.dart';
 import '../services/recovery_sheet_service.dart';
 import '../services/share_catalog_service.dart';
 import '../services/social_service.dart';
+import '../services/social_settings.dart';
 
 /// The app's service locator. Kept to the composition root — widgets get their
 /// repositories from here (or via BlocProvider), never by reaching into it deep
@@ -83,6 +84,7 @@ Future<void> configureDependencies() async {
     ..registerSingleton<LabelTextExtractor>(labelExtractor)
     ..registerSingleton<OnboardingStore>(OnboardingStore(secretStore))
     ..registerSingleton<SocialService>(socialService)
+    ..registerSingleton<SocialSettings>(SocialSettings(secretStore))
     ..registerSingleton<ExportImportService>(
       ExportImportService(
         repository: varietyRepository,