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/lib/ui/your_people_screen.dart

@@ -6,6 +6,7 @@ import 'package:go_router/go_router.dart';
 
 import '../i18n/strings.g.dart';
 import '../services/profile_cache.dart';
+import 'peer_avatar.dart';
 import '../services/social_connection.dart';
 import '../services/social_service.dart';
 import 'theme.dart';
@@ -151,7 +152,9 @@ class _YourPeopleScreenState extends State<YourPeopleScreen> {
                       for (final peer in _youVouchFor)
                         _PersonTile(
                           key: Key('yourPeople.given.$peer'),
+                          pubkey: peer,
                           name: _names[peer] ?? shortPubkey(peer),
+                          cache: widget.profileCache,
                           onOpen: () => context.push('/chat/$peer'),
                           trailing: TextButton(
                             onPressed: _busy ? null : () => _revoke(peer),
@@ -166,7 +169,9 @@ class _YourPeopleScreenState extends State<YourPeopleScreen> {
                       for (final peer in _vouchForYou)
                         _PersonTile(
                           key: Key('yourPeople.received.$peer'),
+                          pubkey: peer,
                           name: _names[peer] ?? shortPubkey(peer),
+                          cache: widget.profileCache,
                           onOpen: () => context.push('/chat/$peer'),
                         ),
                   ],
@@ -217,20 +222,26 @@ class _PersonTile extends StatelessWidget {
   const _PersonTile({
     required this.name,
     required this.onOpen,
+    required this.pubkey,
+    this.cache,
     this.trailing,
     super.key,
   });
 
+  final String pubkey;
   final String name;
+  final ProfileCache? cache;
   final VoidCallback onOpen;
   final Widget? trailing;
 
   @override
   Widget build(BuildContext context) {
     return ListTile(
-      leading: const CircleAvatar(
-        backgroundColor: seedAvatar,
-        child: Icon(Icons.person, color: seedOnAvatar),
+      leading: CachedAvatar(
+        pubkey: pubkey,
+        name: name,
+        cache: cache,
+        radius: 20,
       ),
       title: Text(name),
       trailing: trailing,