- UI display: display_name first

- Fallback: name
  - Technical identity: still name

  ### What changed

  - Added account display helpers and display_name state in user model:
      - flutter/lib/models/user_model.dart:16
  - Account/logout label now uses display_name (@name) when both exist:
      - flutter/lib/mobile/pages/settings_page.dart:689
      - flutter/lib/desktop/pages/desktop_setting_page.dart:2016
      - flutter/lib/desktop/pages/desktop_setting_page.dart:2135
  - Desktop Account info now shows both when applicable:
      - Display Name: ...
      - Username: ...
      - flutter/lib/desktop/pages/desktop_setting_page.dart:2039
  - Previously done group-list behavior remains:
      - group user list displays display_name with name fallback
      - flutter/lib/common/widgets/my_group.dart:187
  - Persistence path for display_name remains enabled (including group cache/submodule field):
      - libs/hbb_common/src/config.rs:2347
  - src/client.rs:2630
  - LoginRequest.my_name now resolves as:
      1. OPTION_DISPLAY_NAME (manual override)
      2. user_info.display_name
      3. user_info.name
      4. OS username fallback

flutter/lib/models/user_model.dart

@@ -16,9 +16,23 @@ bool refreshingUser = false;
 
 class UserModel {
   final RxString userName = ''.obs;
+  final RxString displayName = ''.obs;
   final RxBool isAdmin = false.obs;
   final RxString networkError = ''.obs;
   bool get isLogin => userName.isNotEmpty;
+  String get displayNameOrUserName =>
+      displayName.value.trim().isEmpty ? userName.value : displayName.value;
+  String get accountLabelWithHandle {
+    final username = userName.value.trim();
+    if (username.isEmpty) {
+      return '';
+    }
+    final preferred = displayName.value.trim();
+    if (preferred.isEmpty || preferred == username) {
+      return username;
+    }
+    return '$preferred (@$username)';
+  }
   WeakReference<FFI> parent;
 
   UserModel(this.parent) {
@@ -98,7 +112,8 @@ class UserModel {
   _updateLocalUserInfo() {
     final userInfo = getLocalUserInfo();
     if (userInfo != null) {
-      userName.value = userInfo['name'];
+      userName.value = (userInfo['name'] ?? '').toString();
+      displayName.value = (userInfo['display_name'] ?? '').toString();
     }
   }
 
@@ -110,10 +125,12 @@ class UserModel {
       await gFFI.groupModel.reset();
     }
     userName.value = '';
+    displayName.value = '';
   }
 
   _parseAndUpdateUser(UserPayload user) {
     userName.value = user.name;
+    displayName.value = user.displayName;
     isAdmin.value = user.isAdmin;
     bind.mainSetLocalOption(key: 'user_info', value: jsonEncode(user));
     if (isWeb) {