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/ui/peer_avatar_test.dart

@@ -1,8 +1,20 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/ui/avatar.dart';
 import 'package:tane/ui/peer_avatar.dart';
+import 'package:tane/ui/seed_glyph.dart';
 
 void main() {
+  group('avatar value scheme', () {
+    test('every offered illustration maps to a glyph; unknown → null', () {
+      for (final name in avatarGlyphNames) {
+        expect(avatarGlyphChar(name), isNotNull, reason: name);
+      }
+      expect(avatarGlyphChar('nope'), isNull);
+      expect(avatarGlyphToken('jars'), 'tane:seed:jars');
+    });
+  });
+
   group('peerAvatarColor', () {
     test('is deterministic for the same pubkey', () {
       expect(peerAvatarColor('ab' * 32), peerAvatarColor('ab' * 32));
@@ -34,5 +46,14 @@ void main() {
       expect(find.byIcon(Icons.person_outline), findsOneWidget);
       expect(find.byType(Text), findsNothing);
     });
+
+    testWidgets('renders a seed illustration when the avatar is a token', (
+      tester,
+    ) async {
+      await tester.pumpWidget(host(const PeerAvatar(
+          pubkey: 'aa', name: 'rosa', picture: 'tane:seed:jars')));
+      expect(find.byType(SeedGlyph), findsOneWidget);
+      expect(find.text('R'), findsNothing); // the initial isn't used
+    });
   });
 }