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/test/services/profile_cache_test.dart

@@ -0,0 +1,22 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/services/profile_cache.dart';
+
+import '../support/test_support.dart';
+
+void main() {
+  test('unknown pubkey has no cached name', () async {
+    final cache = ProfileCache(InMemorySecretStore());
+    expect(await cache.name('abc'), isNull);
+  });
+
+  test('setName then name round-trips (trimmed)', () async {
+    final cache = ProfileCache(InMemorySecretStore());
+    await cache.setName('abc', '  Alicia  ');
+    expect(await cache.name('abc'), 'Alicia');
+  });
+
+  test('shortPubkey abbreviates long keys, keeps short ones', () {
+    expect(shortPubkey('abcdef1234567890'), 'abcdef…7890');
+    expect(shortPubkey('short'), 'short');
+  });
+}