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/repositories/auth_repository.dart

@@ -1,7 +1,14 @@
+import 'package:flutter/foundation.dart';
 import 'package:dio/dio.dart';
 import '../core/api/api_client.dart';
 import '../core/auth/auth_state.dart';
 
+typedef AuthLoginFn = Future<AuthUser> Function({
+  required String shopCode,
+  required String username,
+  required String password,
+});
+
 class AuthException implements Exception {
   final String message;
   const AuthException(this.message);
@@ -10,6 +17,9 @@ class AuthException implements Exception {
 }
 
 class AuthRepository {
+  @visibleForTesting
+  static AuthLoginFn? loginOverride;
+
   /// POST /api/v1/auth/login
   /// Request:  { shop_code, username, password }
   /// Response: { data: { access_token, refresh_token, expires_in, user: { id, username, real_name, role } } }
@@ -18,6 +28,14 @@ class AuthRepository {
     required String username,
     required String password,
   }) async {
+    final override = loginOverride;
+    if (override != null) {
+      return override(
+        shopCode: shopCode,
+        username: username,
+        password: password,
+      );
+    }
     try {
       final resp = await PublicApiClient.post('/auth/login', data: {
         'shop_code': shopCode,