feat(client): 网络恢复自动刷新 + 离线嗅探 + 连通性感知登录

网络嗅探与自动恢复：
- ConnectivityNotifier：在线时 30s 检测，离线时切换为 1min 嗅探
- 新增 networkRecoveryCountProvider：网络恢复时自增，各数据 provider
  通过 ref.watch 实现自动重新加载
- inventory/product/stock_in/stock_out/partner provider 均已接入

FutureBuilder 类屏幕：
- finance_screen + batch_tracking_screen 通过 ref.listen 监听恢复事件

修复问题 1：API 超时 10s/30s → 5s/15s，减少无网络时等待
修复问题 2：offline banner 文字由"写操作已禁用"改为实际行为描述

连通性感知：
- app 启动时（_AppBootstrap）立即 forceCheck，路由跳转前状态已就绪
- 登录页：watch connectivityProvider，离线时显示警告 banner
- 登录按钮：离线状态下直接提示，不等待超时

测试：
- login_screen_test + login_flow_test 通过 ConnectivityNotifier(skipInit: true)
  覆盖 provider，避免测试环境中 pending HTTP timer

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

client/lib/screens/auth/login_screen.dart

@@ -6,6 +6,7 @@ import 'package:go_router/go_router.dart';
 import '../../core/auth/auth_state.dart';
 import '../../core/theme/app_theme.dart';
 import '../../core/storage/login_history.dart';
+import '../../providers/connectivity_provider.dart';
 import '../../repositories/auth_repository.dart';
 
 class LoginScreen extends ConsumerStatefulWidget {
@@ -195,6 +196,14 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
 
   Future<void> _login() async {
     if (!_formKey.currentState!.validate()) return;
+
+    // 登录前先检查连通性
+    final isOnline = ref.read(connectivityProvider);
+    if (!isOnline) {
+      setState(() => _errorMessage = '服务器不可达，请检查网络连接后重试');
+      return;
+    }
+
     setState(() {
       _loading = true;
       _errorMessage = null;
@@ -382,6 +391,38 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
                               onFieldSubmitted: (_) => _login(),
                             ),
 
+                            // 离线提示（connectivityProvider 在 _AppBootstrap 中已预热）
+                            if (!ref.watch(connectivityProvider)) ...[
+                              const SizedBox(height: 12),
+                              Container(
+                                width: double.infinity,
+                                padding: const EdgeInsets.symmetric(
+                                    horizontal: 12, vertical: 10),
+                                decoration: BoxDecoration(
+                                  color: const Color(0xFFFFF8E1),
+                                  borderRadius: BorderRadius.circular(4),
+                                  border: Border.all(
+                                      color: const Color(0xFFF57F17)
+                                          .withAlpha(120)),
+                                ),
+                                child: const Row(
+                                  children: [
+                                    Icon(Icons.wifi_off,
+                                        color: Color(0xFFF57F17), size: 16),
+                                    SizedBox(width: 8),
+                                    Expanded(
+                                      child: Text(
+                                        '服务器不可达，请检查网络连接',
+                                        style: TextStyle(
+                                            color: Color(0xFF5D4037),
+                                            fontSize: 13),
+                                      ),
+                                    ),
+                                  ],
+                                ),
+                              ),
+                            ],
+
                             // Inline error
                             if (_errorMessage != null) ...[
                               const SizedBox(height: 12),

client/lib/screens/finance/finance_screen.dart

@@ -6,6 +6,7 @@ import '../../providers/finance_provider.dart';
 import '../../widgets/data_table_card.dart';
 import '../../widgets/multi_select_dropdown.dart' show ColDef, ColumnToggleButton, FilterableColumnHeader;
 import '../../widgets/page_scaffold.dart';
+import '../../providers/connectivity_provider.dart';
 
 class FinanceScreen extends ConsumerWidget {
   const FinanceScreen({super.key});
@@ -101,6 +102,9 @@ class _FinanceTabState extends ConsumerState<_FinanceTab> {
 
   @override
   Widget build(BuildContext context) {
+    // 网络恢复时自动刷新
+    ref.listen(networkRecoveryCountProvider, (_, __) => _refetch());
+
     return FutureBuilder<List<FinanceRecord>>(
       future: _future,
       builder: (context, snap) {

client/lib/screens/inventory/batch_tracking_screen.dart

@@ -5,6 +5,7 @@ import '../../models/inventory.dart';
 import '../../providers/inventory_provider.dart';
 import '../../widgets/data_table_card.dart';
 import '../../widgets/multi_select_dropdown.dart' show ColDef, ColumnToggleButton, FilterableColumnHeader;
+import '../../providers/connectivity_provider.dart';
 
 class BatchTrackingScreen extends ConsumerStatefulWidget {
   const BatchTrackingScreen({super.key});
@@ -77,6 +78,9 @@ class _BatchTrackingScreenState extends ConsumerState<BatchTrackingScreen> {
 
   @override
   Widget build(BuildContext context) {
+    // 网络恢复时自动刷新
+    ref.listen(networkRecoveryCountProvider, (_, __) => _refetch());
+
     return FutureBuilder<List<ProductTrackingRecord>>(
       future: _future,
       builder: (context, snap) {

client/lib/screens/shell/app_shell.dart

@@ -274,7 +274,7 @@ class _AppShellState extends ConsumerState<AppShell> {
                                   size: 16, color: Colors.white),
                               SizedBox(width: 8),
                               Text(
-                                '网络连接已断开 · 当前处于只读模式，所有写操作已禁用',
+                                '网络连接已断开 · 当前显示离线缓存数据，恢复后将自动刷新',
                                 style: TextStyle(
                                     color: Colors.white,
                                     fontSize: 13,