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/peer_avatar.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 
 import '../services/offer_thumbnail.dart' show decodeDataUri;
+import '../services/profile_cache.dart';
 import 'avatar.dart';
 import 'seed_glyph.dart';
 
@@ -71,6 +72,43 @@ class PeerAvatar extends StatelessWidget {
   }
 }
 
+/// A [PeerAvatar] that looks the person's published avatar up from the
+/// [ProfileCache] (their kind:0 `picture`), falling back to the coloured-initial
+/// disc while it loads or when none is cached / no cache is available. Use at
+/// list/row sites (one avatar each); for many avatars of the same few people
+/// (chat bubbles) resolve the picture once into state instead.
+class CachedAvatar extends StatelessWidget {
+  const CachedAvatar({
+    required this.pubkey,
+    this.name,
+    this.cache,
+    this.radius = 14,
+    super.key,
+  });
+
+  final String pubkey;
+  final String? name;
+  final ProfileCache? cache;
+  final double radius;
+
+  @override
+  Widget build(BuildContext context) {
+    final c = cache;
+    if (c == null) {
+      return PeerAvatar(pubkey: pubkey, name: name, radius: radius);
+    }
+    return FutureBuilder<String?>(
+      future: c.picture(pubkey),
+      builder: (context, snap) => PeerAvatar(
+        pubkey: pubkey,
+        name: name,
+        picture: snap.data,
+        radius: radius,
+      ),
+    );
+  }
+}
+
 /// A deterministic, readable avatar colour derived from [pubkey]. Hashes the
 /// key to a hue, then fixes saturation/lightness so white text stays legible on
 /// top. Pure and stable — exposed for testing.