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/core/storage/login_history.dart

@@ -0,0 +1,36 @@
+import 'package:shared_preferences/shared_preferences.dart';
+
+/// Stores up to [_maxItems] recently used hotel codes and usernames.
+/// List is ordered most-recent-first; least-recently-used entry is evicted
+/// when the list exceeds the max.
+class LoginHistoryStorage {
+  static const int _maxItems = 5;
+  static const String _hotelKey = 'login_history_hotels';
+  static const String _usernameKey = 'login_history_usernames';
+
+  static Future<List<String>> getHotelCodes() async {
+    final prefs = await SharedPreferences.getInstance();
+    return prefs.getStringList(_hotelKey) ?? [];
+  }
+
+  static Future<List<String>> getUsernames() async {
+    final prefs = await SharedPreferences.getInstance();
+    return prefs.getStringList(_usernameKey) ?? [];
+  }
+
+  /// Call after a successful login to persist both values.
+  static Future<void> record(String hotelCode, String username) async {
+    final prefs = await SharedPreferences.getInstance();
+    await _push(prefs, _hotelKey, hotelCode);
+    await _push(prefs, _usernameKey, username);
+  }
+
+  static Future<void> _push(
+      SharedPreferences prefs, String key, String value) async {
+    final list = prefs.getStringList(key) ?? [];
+    list.remove(value); // move to front if already present
+    list.insert(0, value);
+    if (list.length > _maxItems) list.removeLast();
+    await prefs.setStringList(key, list);
+  }
+}