feat(chat): sender avatar on both sides, coloured per person

Feedback: avatars only on the left, all the same colour, so it was hard
to tell who said what. In a 1:1 only the peer had an avatar (one colour).
Now each message carries its sender's avatar on their own side — mine
trailing, the peer's leading — each tinted by a deterministic colour from
their pubkey. Two sides, two colours: who-said-what is obvious at a glance.

apps/app_seeds/lib/ui/chat_screen.dart

@@ -302,6 +302,7 @@ class ChatMessageList extends StatelessWidget {
                 message: message,
                 mine: cubit.isMine(message),
                 peerName: peerName,
+                selfPubkey: cubit.selfPubkey,
               ),
             };
           },
@@ -408,32 +409,44 @@ class _DaySeparator extends StatelessWidget {
   }
 }
 
-/// One message line: my messages align to the trailing edge with no avatar;
+/// One message line. Each sender gets their own coloured avatar on their side —
-/// the peer's align to the leading edge with their avatar (1:1 convention).
+/// mine on the trailing edge, the peer's on the leading edge — so at a glance
+/// the two sides are two people, not one column of look-alike bubbles.
 class _MessageRow extends StatelessWidget {
-  const _MessageRow({required this.message, required this.mine, this.peerName});
+  const _MessageRow({
+    required this.message,
+    required this.mine,
+    this.peerName,
+    this.selfPubkey,
+  });
 
   final PrivateMessage message;
   final bool mine;
   final String? peerName;
 
+  /// My own pubkey, so my messages carry my (distinctly coloured) avatar.
+  final String? selfPubkey;
+
   @override
   Widget build(BuildContext context) {
-    if (mine) {
+    // My avatar has no name (it's me) → a glyph on my colour; the peer's shows
-      return Align(
+    // their initial on theirs. Different pubkeys → different colours.
-        alignment: AlignmentDirectional.centerEnd,
+    final avatar = mine
-        child: _Bubble(message: message, mine: true),
+        ? PeerAvatar(pubkey: selfPubkey ?? message.fromPubkey)
-      );
+        : PeerAvatar(pubkey: message.fromPubkey, name: peerName);
-    }
+    final bubble = Flexible(
+      child: _Bubble(message: message, mine: mine),
+    );
     return Padding(
       padding: const EdgeInsets.symmetric(vertical: 4),
       child: Row(
         crossAxisAlignment: CrossAxisAlignment.end,
-        children: [
+        mainAxisAlignment: mine
-          PeerAvatar(pubkey: message.fromPubkey, name: peerName),
+            ? MainAxisAlignment.end
-          const SizedBox(width: 8),
+            : MainAxisAlignment.start,
-          Flexible(child: _Bubble(message: message, mine: false)),
+        children: mine
-        ],
+            ? [bubble, const SizedBox(width: 8), avatar]
+            : [avatar, const SizedBox(width: 8), bubble],
       ),
     );
   }

apps/app_seeds/test/ui/chat_message_list_test.dart

@@ -113,9 +113,7 @@ void main() {
     expect(find.byKey(const Key('chat.daySeparator')), findsNWidgets(2));
   });
 
-  testWidgets('the peer bubble carries an avatar, mine does not', (
+  testWidgets('each sender gets a distinctly coloured avatar', (tester) async {
-    tester,
-  ) async {
     final transport = _FakeMessageTransport();
     final cubit = MessagesCubit(transport, peerPubkey: peer, selfPubkey: me)
       ..start();
@@ -126,8 +124,14 @@ void main() {
     await tester.pumpWidget(host(cubit));
     await tester.pump();
 
-    // One avatar, for the single peer message; my own bubble has none.
+    // Both sides carry an avatar, and the two people are different colours —
-    expect(find.byType(PeerAvatar), findsOneWidget);
+    // so you can tell who said what at a glance.
+    expect(find.byType(PeerAvatar), findsNWidgets(2));
+    final discs = tester
+        .widgetList<CircleAvatar>(find.byType(CircleAvatar))
+        .toList();
+    expect(discs, hasLength(2));
+    expect(discs.first.backgroundColor, isNot(discs.last.backgroundColor));
   });
 
   testWidgets('a new message lands in view at the bottom, not below the fold', (