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/test/services/profile_cache_test.dart

@@ -15,6 +15,14 @@ void main() {
     expect(await cache.name('abc'), 'Alicia');
   });
 
+  test('picture caches independently of name', () async {
+    final cache = ProfileCache(InMemorySecretStore());
+    expect(await cache.picture('abc'), isNull);
+    await cache.setPicture('abc', '  tane:seed:jars  ');
+    expect(await cache.picture('abc'), 'tane:seed:jars');
+    expect(await cache.name('abc'), isNull); // name untouched
+  });
+
   test('shortPubkey abbreviates long keys, keeps short ones', () {
     expect(shortPubkey('abcdef1234567890'), 'abcdef…7890');
     expect(shortPubkey('short'), 'short');

apps/app_seeds/test/services/profile_store_test.dart

@@ -17,4 +17,10 @@ void main() {
     expect(await store.name(), 'Alicia');
     expect(await store.about(), 'tomate rosa');
   });
+
+  test('avatar defaults empty and round-trips', () async {
+    expect(await store.avatar(), '');
+    await store.save(name: 'A', about: '', avatar: 'tane:seed:jars');
+    expect(await store.avatar(), 'tane:seed:jars');
+  });
 }