feat(block2): show contact names (resolve peers' published profiles)

Closes the profile loop — peers now appear by name, not a raw key.
- ProfileCache: keystore-backed cache of peers' published display names (works
  offline) + shortPubkey() fallback.
- Chat: the app bar shows the peer's name (cached first, then freshened from
  their kind:0 over the session); falls back to a short key.
- Inbox: each conversation shows the cached name, and missing names are resolved
  over one session and cached.
- Wired via DI + TaneApp(profileCache).

Analyzer clean; ProfileCache covered by a plain test.

apps/app_seeds/lib/main.dart

@@ -10,6 +10,7 @@ import 'services/coarse_location.dart';
 import 'services/message_store.dart';
 import 'services/offer_outbox.dart';
 import 'services/onboarding_store.dart';
+import 'services/profile_cache.dart';
 import 'services/profile_store.dart';
 import 'services/social_service.dart';
 import 'services/social_settings.dart';
@@ -31,6 +32,7 @@ Future<void> main() async {
         outbox: getIt<OfferOutbox>(),
         messageStore: getIt<MessageStore>(),
         profileStore: getIt<ProfileStore>(),
+        profileCache: getIt<ProfileCache>(),
         showIntro: !await onboarding.introSeen(),
         autoBackup: getIt.isRegistered<AutoBackupService>()
             ? getIt<AutoBackupService>()