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/see/group/GroupEditorState.dart

@@ -245,7 +245,7 @@ class GroupEditorState extends State<GroupEditor>
           text(name, 26, TextType.Light,
               color: colorWhite, maxLines: 2, overflow: TextOverflow.ellipsis),
           const SizedBox(height: 16),
-          avatarStack(const <String>["Alex"], 34),
+          avatarStack(const <String>["You"], 34),
         ],
       ),
     );