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_cache.dart

@@ -14,6 +14,10 @@ class ProfileCache {
   final SecretStore _store;
   final String _prefix;
 
+  // Avatars peers have published (kind:0 `picture`) live under a parallel key
+  // namespace, so a name and an avatar are cached independently.
+  String get _picPrefix => '${_prefix.substring(0, _prefix.length - 5)}picture.';
+
   /// The cached name for [pubkeyHex], or null if none is known.
   Future<String?> name(String pubkeyHex) async {
     final value = await _store.read('$_prefix$pubkeyHex');
@@ -22,6 +26,16 @@ class ProfileCache {
 
   Future<void> setName(String pubkeyHex, String name) =>
       _store.write('$_prefix$pubkeyHex', name.trim());
+
+  /// The cached avatar for [pubkeyHex] (a `data:` photo or `tane:seed:<glyph>`
+  /// token), or null if none is known.
+  Future<String?> picture(String pubkeyHex) async {
+    final value = await _store.read('$_picPrefix$pubkeyHex');
+    return (value == null || value.isEmpty) ? null : value;
+  }
+
+  Future<void> setPicture(String pubkeyHex, String picture) =>
+      _store.write('$_picPrefix$pubkeyHex', picture.trim());
 }
 
 /// A compact, human-ish rendering of a public key when no name is known yet.