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/profile_store.dart

@@ -17,6 +17,7 @@ class ProfileStore {
   String get _nameKey => '${_base}name';
   String get _aboutKey => '${_base}about';
   String get _g1Key => '${_base}g1';
+  String get _avatarKey => '${_base}avatar';
 
   Future<String> name() async => (await _store.read(_nameKey)) ?? '';
   Future<String> about() async => (await _store.read(_aboutKey)) ?? '';
@@ -24,13 +25,20 @@ class ProfileStore {
   /// Your Ğ1 (Duniter) address, if you chose to share one.
   Future<String> g1() async => (await _store.read(_g1Key)) ?? '';
 
+  /// Your avatar: either a `data:image/jpeg;base64,…` photo thumbnail or a
+  /// `tane:seed:<glyph>` illustration token (see `ui/avatar.dart`). Empty for
+  /// the default coloured-initial disc. Published as the kind:0 `picture`.
+  Future<String> avatar() async => (await _store.read(_avatarKey)) ?? '';
+
   Future<void> save({
     required String name,
     required String about,
     String g1 = '',
+    String avatar = '',
   }) async {
     await _store.write(_nameKey, name.trim());
     await _store.write(_aboutKey, about.trim());
     await _store.write(_g1Key, g1.trim());
+    await _store.write(_avatarKey, avatar.trim());
   }
 }