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/GroupChatState.dart

@@ -98,6 +98,16 @@ class GroupChatState extends State<GroupChat> implements ConnectGroupChat {
     _onSend(reply);
   }
 
+  /// A message author's photo, looked up among the group's members.
+  String _photoOf(String authorId) {
+    for (final member in widget.group.members) {
+      if (member.id == authorId) {
+        return member.photo;
+      }
+    }
+    return "";
+  }
+
   // ── Views ─────────────────────────────────────────────────────────────────
 
   Widget _mobileView(BoxConstraints constraints) {
@@ -145,8 +155,7 @@ class GroupChatState extends State<GroupChat> implements ConnectGroupChat {
 
     return Container(
       decoration: BoxDecoration(
-        borderRadius:
-            const BorderRadius.vertical(bottom: Radius.circular(20)),
+        borderRadius: const BorderRadius.vertical(bottom: Radius.circular(20)),
         gradient: LinearGradient(
           begin: Alignment.topLeft,
           end: Alignment.bottomRight,
@@ -188,7 +197,10 @@ class GroupChatState extends State<GroupChat> implements ConnectGroupChat {
           const SizedBox(width: 10),
           avatarStack(
               widget.group.members.map((member) => member.name).toList(), 30,
-              max: 3, border: colorNight),
+              photos:
+                  widget.group.members.map((member) => member.photo).toList(),
+              max: 3,
+              border: colorNight),
         ],
       ),
     );
@@ -275,7 +287,8 @@ class GroupChatState extends State<GroupChat> implements ConnectGroupChat {
         children: [
           grouped
               ? const SizedBox(width: 34)
-              : memberAvatar(message.authorName, 34),
+              : memberAvatar(message.authorName, 34,
+                  photo: _photoOf(message.authorId)),
           const SizedBox(width: 8),
           Flexible(
             child: Container(
@@ -328,7 +341,8 @@ class GroupChatState extends State<GroupChat> implements ConnectGroupChat {
         ),
         child: Row(
           children: [
-            memberAvatar(message.authorName, 34),
+            memberAvatar(message.authorName, 34,
+                photo: _photoOf(message.authorId)),
             const SizedBox(width: 12),
             Expanded(
               child: Column(