feat(client): 登录历史下拉、macOS 兼容修复、UI 优化

- 替换 flutter_secure_storage 为 shared_preferences，解决 macOS Keychain 签名报错
- 登录页门店编号/用户名支持历史下拉（最多5条，LRU 淘汰），使用 Overlay + CompositedTransformFollower 实现浮层
- logout 仅删除 auth token，保留登录历史
- AppShell 时钟抽为独立 _ClockWidget，避免每秒 setState 导致 TabBarView 叠影
- ShellRoute 子路由改用 NoTransitionPage，消除切换页面时的过渡动画叠影
- 新增 auth_state_test、auth_repository_test、login_screen_test 共 14 个单元/Widget 测试

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/screens/shell/app_shell.dart

@@ -16,8 +16,6 @@ class AppShell extends ConsumerStatefulWidget {
 
 class _AppShellState extends ConsumerState<AppShell> {
   bool _sidebarExpanded = true;
-  late Timer _timer;
-  late String _currentTime;
   final String _loginTime =
       DateFormat('HH:mm:ss').format(DateTime.now());
 
@@ -34,24 +32,6 @@ class _AppShellState extends ConsumerState<AppShell> {
     _NavItem(icon: Icons.settings, label: '系统设置', path: '/settings'),
   ];
 
-  @override
-  void initState() {
-    super.initState();
-    _currentTime = DateFormat('HH:mm:ss').format(DateTime.now());
-    _timer = Timer.periodic(const Duration(seconds: 1), (_) {
-      if (mounted) {
-        setState(() =>
-            _currentTime = DateFormat('HH:mm:ss').format(DateTime.now()));
-      }
-    });
-  }
-
-  @override
-  void dispose() {
-    _timer.cancel();
-    super.dispose();
-  }
-
   @override
   Widget build(BuildContext context) {
     final user = ref.watch(authStateProvider).user;
@@ -192,9 +172,7 @@ class _AppShellState extends ConsumerState<AppShell> {
                                   text: '登录时间：$_loginTime'),
                               const _StatusDivider(),
                             ],
-                            _StatusItem(
-                                icon: Icons.access_time,
-                                text: '当前时间：$_currentTime'),
+                            const _ClockWidget(),
                             const Spacer(),
                             const _StatusItem(
                                 icon: Icons.info_outline,
@@ -325,3 +303,34 @@ class _StatusDivider extends StatelessWidget {
     );
   }
 }
+
+class _ClockWidget extends StatefulWidget {
+  const _ClockWidget();
+  @override
+  State<_ClockWidget> createState() => _ClockWidgetState();
+}
+
+class _ClockWidgetState extends State<_ClockWidget> {
+  late Timer _timer;
+  late String _time;
+
+  @override
+  void initState() {
+    super.initState();
+    _time = DateFormat('HH:mm:ss').format(DateTime.now());
+    _timer = Timer.periodic(const Duration(seconds: 1), (_) {
+      if (mounted) setState(() => _time = DateFormat('HH:mm:ss').format(DateTime.now()));
+    });
+  }
+
+  @override
+  void dispose() {
+    _timer.cancel();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return _StatusItem(icon: Icons.access_time, text: '当前时间：$_time');
+  }
+}