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/macos/Podfile.lock

@@ -1,21 +1,22 @@
 PODS:
-  - flutter_secure_storage_macos (6.1.3):
-    - FlutterMacOS
   - FlutterMacOS (1.0.0)
+  - shared_preferences_foundation (0.0.1):
+    - Flutter
+    - FlutterMacOS
 
 DEPENDENCIES:
-  - flutter_secure_storage_macos (from `Flutter/ephemeral/.symlinks/plugins/flutter_secure_storage_macos/macos`)
   - FlutterMacOS (from `Flutter/ephemeral`)
+  - shared_preferences_foundation (from `Flutter/ephemeral/.symlinks/plugins/shared_preferences_foundation/darwin`)
 
 EXTERNAL SOURCES:
-  flutter_secure_storage_macos:
-    :path: Flutter/ephemeral/.symlinks/plugins/flutter_secure_storage_macos/macos
   FlutterMacOS:
     :path: Flutter/ephemeral
+  shared_preferences_foundation:
+    :path: Flutter/ephemeral/.symlinks/plugins/shared_preferences_foundation/darwin
 
 SPEC CHECKSUMS:
-  flutter_secure_storage_macos: 7f45e30f838cf2659862a4e4e3ee1c347c2b3b54
   FlutterMacOS: d0db08ddef1a9af05a5ec4b724367152bb0500b1
+  shared_preferences_foundation: 7036424c3d8ec98dfe75ff1667cb0cd531ec82bb
 
 PODFILE CHECKSUM: 54d867c82ac51cbd61b565781b9fada492027009