feat(profile): show other people's avatars in chat, chat list, your people

Follow-up to the avatar feature: peers' published photos/illustrations
now actually render where people appear.

- CachedAvatar: a PeerAvatar that resolves the person's picture from the
  ProfileCache (FutureBuilder), falling back to the coloured-initial disc.
  For list/row sites — one avatar each.
- Chat bubbles: the peer's and my own avatar are resolved once into state
  (_peerPicture/_selfPicture — from the cache / ProfileStore, freshened
  from the peer's published profile) and threaded to each bubble, so many
  bubbles of two people don't each hit the cache.
- Chat list + your-people rows: swapped the static person-icon disc for
  CachedAvatar, so photos/illustrations show there too.
- Tests: CachedAvatar render modes (cached illustration / initial / no
  cache). analyze clean; chat + your-people suites green.

Market offers show no author avatar today (would be a new element, not a
swap) — left out of scope.

apps/app_seeds/test/ui/peer_avatar_test.dart

@@ -1,9 +1,12 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/services/profile_cache.dart';
 import 'package:tane/ui/avatar.dart';
 import 'package:tane/ui/peer_avatar.dart';
 import 'package:tane/ui/seed_glyph.dart';
 
+import '../support/test_support.dart';
+
 void main() {
   group('avatar value scheme', () {
     test('every offered illustration maps to a glyph; unknown → null', () {
@@ -56,4 +59,34 @@ void main() {
       expect(find.text('R'), findsNothing); // the initial isn't used
     });
   });
+
+  group('CachedAvatar', () {
+    Widget host(Widget child) => MaterialApp(
+      home: Scaffold(body: Center(child: child)),
+    );
+
+    testWidgets('paints the cached illustration for the peer', (tester) async {
+      final cache = ProfileCache(InMemorySecretStore());
+      await cache.setPicture('peer1', 'tane:seed:sack');
+      await tester.pumpWidget(host(
+          CachedAvatar(pubkey: 'peer1', name: 'Bea', cache: cache)));
+      await tester.pumpAndSettle();
+      expect(find.byType(SeedGlyph), findsOneWidget);
+    });
+
+    testWidgets('falls back to the initial when nothing is cached', (
+      tester,
+    ) async {
+      final cache = ProfileCache(InMemorySecretStore());
+      await tester.pumpWidget(host(
+          CachedAvatar(pubkey: 'peer2', name: 'Bea', cache: cache)));
+      await tester.pumpAndSettle();
+      expect(find.text('B'), findsOneWidget);
+    });
+
+    testWidgets('no cache → the coloured-initial disc', (tester) async {
+      await tester.pumpWidget(host(const CachedAvatar(pubkey: 'x', name: 'Bea')));
+      expect(find.text('B'), findsOneWidget);
+    });
+  });
 }