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/test/widget_test.dart

@@ -1,30 +1,5 @@
-// This is a basic Flutter widget test.
-//
-// To perform an interaction with a widget in your test, use the WidgetTester
-// utility in the flutter_test package. For example, you can send tap and scroll
-// gestures. You can also use WidgetTester to find child widgets in the widget
-// tree, read text, and verify that the values of widget properties are correct.
-
-import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-import 'package:jiu_client/main.dart';
-
 void main() {
-  testWidgets('Counter increments smoke test', (WidgetTester tester) async {
-    // Build our app and trigger a frame.
-    await tester.pumpWidget(const MyApp());
-
-    // Verify that our counter starts at 0.
-    expect(find.text('0'), findsOneWidget);
-    expect(find.text('1'), findsNothing);
-
-    // Tap the '+' icon and trigger a frame.
-    await tester.tap(find.byIcon(Icons.add));
-    await tester.pump();
-
-    // Verify that our counter has incremented.
-    expect(find.text('0'), findsNothing);
-    expect(find.text('1'), findsOneWidget);
-  });
+  test('placeholder', () => expect(true, isTrue));
 }