Show real photos for people instead of scene art

Your avatar is your photo, picked from the device by tapping it on Home or
from a new Profile photo row in Settings, shrunk to 256 px and kept on the
phone (there is no upload endpoint yet). Group members show the photo the
server sends in a new `photo` field, everywhere avatars appear. Without a
photo, an initial on a neutral disc. Mock mode bundles free Unsplash
portraits for the sample people.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KhQh614DMeYoQEH9SoMRUw

frontend/lib/Grounded/about/external/data/GroupMember.dart

@@ -4,6 +4,9 @@ class GroupMember {
 
   String name;
 
+  /// Their profile photo (URL), or empty for none.
+  String photo;
+
   /// "Owner" or "Member".
   String role;
 
@@ -24,6 +27,7 @@ class GroupMember {
   GroupMember({
     this.id = "",
     this.name = "",
+    this.photo = "",
     this.role = "Member",
     this.keptRate = 0,
     this.streak = 0,
@@ -38,6 +42,7 @@ class GroupMember {
     return GroupMember(
       id: json['id'] ?? "",
       name: json['name'] ?? "",
+      photo: json['photo'] ?? "",
       role: json['role'] ?? "Member",
       keptRate: (json['keptRate'] ?? 0).toDouble(),
       streak: json['streak'] ?? 0,
@@ -53,6 +58,7 @@ class GroupMember {
     final Map<String, dynamic> data = <String, dynamic>{};
     data['id'] = id;
     data['name'] = name;
+    data['photo'] = photo;
     data['role'] = role;
     data['keptRate'] = keptRate;
     data['streak'] = streak;