feat(profile): default avatars via DiceBear "thumbs" (CC0), drop boring_avatars

Replace the flutter_boring_avatars fallback with a friendly DiceBear
"thumbs" face, generated offline (dicebear_core) and deterministically
from the pubkey, rendered as SVG (flutter_svg). The thumbs style is
CC0 1.0 (public domain) — no attribution, AGPL-clean — unlike Multiavatar
whose custom licence restricts use. Style parsed once and per-pubkey SVG
memoized; the CC0 <metadata> block is stripped to keep flutter_svg quiet.

apps/app_seeds/lib/ui/peer_avatar.dart

@@ -1,27 +1,18 @@
+import 'package:dicebear_core/dicebear_core.dart' show Avatar, Style;
+import 'package:dicebear_styles/thumbs.dart';
 import 'package:flutter/material.dart';
-import 'package:flutter_boring_avatars/flutter_boring_avatars.dart';
+import 'package:flutter_svg/flutter_svg.dart';
 
 import '../services/offer_thumbnail.dart' show decodeDataUri;
 import '../services/profile_cache.dart';
 import 'avatar.dart';
 import 'seed_glyph.dart';
-import 'theme.dart';
-
-/// Palette for the generated default avatars — earthy seed-and-leaf tones so
-/// the fallback art sits inside the app's palette rather than fighting it.
-const _generatedAvatarPalette = BoringAvatarPalette([
-  seedGreen,
-  seedRating, // warm amber
-  Color(0xFFB5541F), // terracotta pot
-  Color(0xFF6B4F2A), // seed-coat brown
-  Color(0xFFD3E8C8), // pale leaf
-]);
 
 /// A small round avatar for a person. When they've set an avatar ([picture] — a
 /// `data:` photo thumbnail or a `tane:seed:<glyph>` illustration) it's shown;
-/// otherwise it falls back to a generated pattern drawn deterministically from
-/// their [pubkey] (boring-avatars "beam"). Same pubkey → same picture on every
-/// device, so a person stays visually recognizable without sharing anything.
+/// otherwise it falls back to a [GeneratedAvatar] drawn deterministically from
+/// their [pubkey]. Same pubkey → same picture on every device, so a person
+/// stays visually recognizable without sharing anything.
 class PeerAvatar extends StatelessWidget {
   const PeerAvatar({
     required this.pubkey,
@@ -34,7 +25,7 @@ class PeerAvatar extends StatelessWidget {
   final String pubkey;
   final String? name;
 
-  /// The person's chosen avatar value; null/empty → the coloured-initial disc.
+  /// The person's chosen avatar value; null/empty → the [GeneratedAvatar].
   final String? picture;
   final double radius;
 
@@ -58,16 +49,52 @@ class PeerAvatar extends StatelessWidget {
       }
     }
 
+    return GeneratedAvatar(pubkey: pubkey, name: name, radius: radius);
+  }
+}
+
+/// The `thumbs` DiceBear style, parsed once — [Style.parse] validates the style
+/// definition against its schema, which is not free to repeat per build.
+final _thumbsStyle = Style.parse(thumbs);
+
+/// Rendered-SVG memo keyed by pubkey, so a person's avatar is generated once and
+/// reused across every rebuild and list row rather than re-rendered each time.
+final _svgCache = <String, String>{};
+
+/// Strips the `<metadata>` block (the style's CC0 licence notice) that
+/// flutter_svg cannot parse and logs a warning for; CC0 requires no attribution
+/// in the rendered image, and the licence still ships with the package source.
+final _metadataElement = RegExp(r'<metadata\b[^>]*>.*?</metadata>', dotAll: true);
+
+String _renderThumbs(String pubkey) =>
+    Avatar(_thumbsStyle, {'seed': pubkey}).svg.replaceAll(_metadataElement, '');
+
+/// The deterministic default avatar for a person with no chosen [picture]: a
+/// friendly DiceBear "thumbs" face (CC0) seeded from their [pubkey], so the same
+/// key yields the same face on every device with nothing shared. Rendered as an
+/// SVG (no network, no assets). Exposes [pubkey] so call sites/tests can find it.
+class GeneratedAvatar extends StatelessWidget {
+  const GeneratedAvatar({
+    required this.pubkey,
+    this.name,
+    this.radius = 14,
+    super.key,
+  });
+
+  final String pubkey;
+  final String? name;
+  final double radius;
+
+  @override
+  Widget build(BuildContext context) {
+    final svg = _svgCache[pubkey] ??= _renderThumbs(pubkey);
     return Semantics(
       label: name,
       image: true,
-      child: SizedBox.square(
-        dimension: radius * 2,
-        child: BoringAvatar(
-          name: pubkey,
-          type: BoringAvatarType.beam,
-          palette: _generatedAvatarPalette,
-          shape: const OvalBorder(),
+      child: ClipOval(
+        child: SizedBox.square(
+          dimension: radius * 2,
+          child: SvgPicture.string(svg, fit: BoxFit.cover),
         ),
       ),
     );