feat(profile): profile photo or seed-illustration avatar

Profiles had only a coloured-initial disc. Now each person can set an
avatar — a real photo OR one of our own seed illustrations (so
pseudonymity stays the default; no photo required).

- ui/avatar.dart: the one-string value scheme carried in the kind:0
  'picture' — a 'data:' photo thumbnail, a 'tane:seed:<glyph>' token, or
  empty. The Nostr ProfileTransport already published 'picture'.
- Photos ride inline as a tiny thumbnail (reuses offerThumbnailDataUri,
  24 KB cap) — no media server, like offer photos.
- avatar_edit.dart: pick/take a photo, choose a seed illustration, or
  remove; ProfileStore stores it; ProfileScreen shows a big editable
  avatar and publishes it.
- PeerAvatar renders photo / illustration / initial fallback.
- ProfileCache gains picture/setPicture; the inbox caches peers' avatars
  alongside their names.
- i18n avatar block (en/es/pt/ast). Tests: value scheme + PeerAvatar
  render modes, store + cache round-trips.

Follow-up: render peers' avatars in chat/market/your-people (thread the
cached picture into PeerAvatar at each call site).

apps/app_seeds/lib/services/inbox_service.dart

@@ -117,12 +117,20 @@ class InboxService {
     final cache = _profileCache;
     final session = _session;
     if (cache == null || session == null) return;
-    if (await cache.name(peerPubkey) != null) return; // already known
+    // Fetch once we're missing either the name or the avatar for this peer.
+    final haveName = await cache.name(peerPubkey) != null;
+    final havePic = await cache.picture(peerPubkey) != null;
+    if (haveName && havePic) return;
     try {
       final profile = await session.profile.fetch(peerPubkey);
-      if (profile != null && profile.name.isNotEmpty) {
-        await cache.setName(peerPubkey, profile.name);
-        if (!_changes.isClosed) _changes.add(null); // re-render with the name
+      if (profile == null) return;
+      if (profile.name.isNotEmpty) await cache.setName(peerPubkey, profile.name);
+      if (profile.picture.isNotEmpty) {
+        await cache.setPicture(peerPubkey, profile.picture);
+      }
+      if ((profile.name.isNotEmpty || profile.picture.isNotEmpty) &&
+          !_changes.isClosed) {
+        _changes.add(null); // re-render with the name/avatar
       }
     } catch (_) {
       // best effort — the short key shows meanwhile