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/router/app_router.dart

@@ -1,3 +1,4 @@
+import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 import '../../screens/auth/login_screen.dart';
@@ -13,6 +14,9 @@ import '../../screens/products/products_screen.dart';
 import '../../screens/settings/settings_screen.dart';
 import '../auth/auth_state.dart';
 
+Page<void> _noTransition(Widget child) =>
+    NoTransitionPage<void>(child: child);
+
 final appRouterProvider = Provider<GoRouter>((ref) {
   final authState = ref.watch(authStateProvider);
 
@@ -35,27 +39,33 @@ final appRouterProvider = Provider<GoRouter>((ref) {
         routes: [
           GoRoute(
               path: '/stock-in',
-              builder: (_, __) => const StockInListScreen()),
+              pageBuilder: (_, __) => _noTransition(const StockInListScreen())),
           GoRoute(
               path: '/stock-in/new',
-              builder: (_, __) => const StockInFormScreen()),
+              pageBuilder: (_, __) => _noTransition(const StockInFormScreen())),
           GoRoute(
               path: '/stock-out',
-              builder: (_, __) => const StockOutListScreen()),
+              pageBuilder: (_, __) => _noTransition(const StockOutListScreen())),
           GoRoute(
               path: '/inventory',
-              builder: (_, __) => const InventoryListScreen()),
+              pageBuilder: (_, __) =>
+                  _noTransition(const InventoryListScreen())),
           GoRoute(
               path: '/inventory/check',
-              builder: (_, __) => const InventoryCheckScreen()),
+              pageBuilder: (_, __) =>
+                  _noTransition(const InventoryCheckScreen())),
           GoRoute(
-              path: '/partners', builder: (_, __) => const PartnersScreen()),
+              path: '/partners',
+              pageBuilder: (_, __) => _noTransition(const PartnersScreen())),
           GoRoute(
-              path: '/finance', builder: (_, __) => const FinanceScreen()),
+              path: '/finance',
+              pageBuilder: (_, __) => _noTransition(const FinanceScreen())),
           GoRoute(
-              path: '/products', builder: (_, __) => const ProductsScreen()),
+              path: '/products',
+              pageBuilder: (_, __) => _noTransition(const ProductsScreen())),
           GoRoute(
-              path: '/settings', builder: (_, __) => const SettingsScreen()),
+              path: '/settings',
+              pageBuilder: (_, __) => _noTransition(const SettingsScreen())),
         ],
       ),
     ],