fix avatar fallback (#14458)

* fix avatar fallback

Signed-off-by: 21pages <sunboeasy@gmail.com>

* fix(ui): improve avatar fallback handling and layout consistency

  - Always show spacing in account section regardless of avatar presence
  - Handle null return from buildAvatarWidget with proper fallback
  - Adjust mobile settings avatar size to 28

Signed-off-by: 21pages <sunboeasy@gmail.com>

---------

Signed-off-by: 21pages <sunboeasy@gmail.com>

flutter/lib/mobile/pages/server_page.dart

@@ -857,16 +857,16 @@ class ClientInfo extends StatelessWidget {
 
   Widget _buildAvatar(BuildContext context) {
     final fallback = CircleAvatar(
-      backgroundColor: str2color(
-          client.name,
+      backgroundColor: str2color(client.name,
           Theme.of(context).brightness == Brightness.light ? 255 : 150),
       child: Text(client.name.isNotEmpty ? client.name[0] : '?'),
     );
     return buildAvatarWidget(
-      avatar: client.avatar,
-      size: 40,
-      fallback: fallback,
-    )!;
+          avatar: client.avatar,
+          size: 40,
+          fallback: fallback,
+        ) ??
+        fallback;
   }
 }