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/core/api/api_client.dart

@@ -8,8 +8,8 @@ import '../../providers/connectivity_provider.dart';
 /// Public Dio instance for unauthenticated calls (login / refresh)
 final _publicDio = Dio(BaseOptions(
   baseUrl: AppConfig.apiBaseUrl,
-  connectTimeout: const Duration(seconds: 10),
-  receiveTimeout: const Duration(seconds: 30),
+  connectTimeout: const Duration(seconds: 5),
+  receiveTimeout: const Duration(seconds: 15),
   headers: {'Content-Type': 'application/json'},
 ));
 
@@ -52,8 +52,8 @@ class ApiClient {
   }) {
     _dio = Dio(BaseOptions(
       baseUrl: AppConfig.apiBaseUrl,
-      connectTimeout: const Duration(seconds: 10),
-      receiveTimeout: const Duration(seconds: 30),
+      connectTimeout: const Duration(seconds: 5),
+      receiveTimeout: const Duration(seconds: 15),
       headers: {
         'Content-Type': 'application/json',
         if (token != null) 'Authorization': 'Bearer $token',