feat(profile): Material default avatar + square photo crop

Drop the DiceBear generated avatar (and dicebear_core/dicebear_styles/
flutter_svg): the default is again a Material coloured-initial disc from
the pubkey. Picking a photo now goes through a square crop step
(crop_your_image — pure Flutter, all platforms incl. desktop, Apache-2.0,
only pulls the image package we already had) before the 24 KB thumbnail.

apps/app_seeds/lib/ui/peer_avatar.dart

@@ -1,7 +1,4 @@
-import 'package:dicebear_core/dicebear_core.dart' show Avatar, Style;
-import 'package:dicebear_styles/thumbs.dart';
 import 'package:flutter/material.dart';
-import 'package:flutter_svg/flutter_svg.dart';
 
 import '../services/offer_thumbnail.dart' show decodeDataUri;
 import '../services/profile_cache.dart';
@@ -9,10 +6,11 @@ import 'avatar.dart';
 import 'seed_glyph.dart';
 
 /// 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 [GeneratedAvatar] drawn deterministically from
-/// their [pubkey]. Same pubkey → same picture on every device, so a person
-/// stays visually recognizable without sharing anything.
+/// `data:` photo thumbnail, or a legacy `tane:seed:<glyph>` illustration) it's
+/// shown; otherwise it falls back to a standard disc coloured deterministically
+/// from their [pubkey], carrying the first letter of their [name] (a person icon
+/// when unknown). Same pubkey → same colour on every device, so a person stays
+/// visually recognizable without sharing anything.
 class PeerAvatar extends StatelessWidget {
   const PeerAvatar({
     required this.pubkey,
@@ -25,7 +23,7 @@ class PeerAvatar extends StatelessWidget {
   final String pubkey;
   final String? name;
 
-  /// The person's chosen avatar value; null/empty → the [GeneratedAvatar].
+  /// The person's chosen avatar value; null/empty → the coloured-initial disc.
   final String? picture;
   final double radius;
 
@@ -49,61 +47,35 @@ 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: ClipOval(
-        child: SizedBox.square(
-          dimension: radius * 2,
-          child: SvgPicture.string(svg, fit: BoxFit.cover),
-        ),
-      ),
+    final letter = _initial(name);
+    return CircleAvatar(
+      radius: radius,
+      backgroundColor: peerAvatarColor(pubkey),
+      child: letter == null
+          ? Icon(Icons.person_outline, size: radius, color: Colors.white)
+          : Text(
+              letter,
+              style: TextStyle(
+                color: Colors.white,
+                fontSize: radius,
+                fontWeight: FontWeight.w600,
+              ),
+            ),
     );
   }
+
+  static String? _initial(String? name) {
+    if (name == null) return null;
+    final trimmed = name.trim();
+    if (trimmed.isEmpty) return null;
+    // characters.first handles emoji/combining marks safely.
+    return trimmed.characters.first.toUpperCase();
+  }
 }
 
 /// A [PeerAvatar] that looks the person's published avatar up from the
-/// [ProfileCache] (their kind:0 `picture`), falling back to the generated
-/// pattern while it loads or when none is cached / no cache is available. Use at
+/// [ProfileCache] (their kind:0 `picture`), falling back to the coloured-initial
+/// disc while it loads or when none is cached / no cache is available. Use at
 /// list/row sites (one avatar each); for many avatars of the same few people
 /// (chat bubbles) resolve the picture once into state instead.
 class CachedAvatar extends StatelessWidget {