- 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/mobile/pages/settings_page.dart

@@ -688,7 +688,7 @@ class _SettingsState extends State<SettingsPage> with WidgetsBindingObserver {
               SettingsTile(
                 title: Obx(() => Text(gFFI.userModel.userName.value.isEmpty
                     ? translate('Login')
-                    : '${translate('Logout')} (${gFFI.userModel.userName.value})')),
+                    : '${translate('Logout')} (${gFFI.userModel.accountLabelWithHandle})')),
                 leading: Icon(Icons.person),
                 onPressed: (context) {
                   if (gFFI.userModel.userName.value.isEmpty) {