feat(client): 初版 Flutter UI 框架，仿照参考截图实现

- 蓝色顶栏（#1565C0）+ 深蓝侧边栏（#0D47A1），含折叠/展开
- 底部状态栏：门店编号、登录用户、登录时间、实时时钟、版本号
- 登录页：居中卡片，支持密码显示/隐藏
- 入库单列表：筛选/搜索/日期选择/分页，三个 Tab（入库单/查询/审核）
- 新建入库单：基本信息 + 商品明细动态增删，实时计算合计金额
- 出库单列表：同入库单风格
- 库存查询：4 张统计卡片 + 颜色高亮缺货/预警行，库存预警 Tab
- 往来单位、财务、商品、系统设置（用户/仓库/编号规则/系统参数）
- 使用 Riverpod 状态管理 + go_router 路由 + Dio HTTP

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

client/lib/core/api/api_client.dart

@@ -0,0 +1,35 @@
+import 'package:dio/dio.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import '../auth/auth_state.dart';
+
+final apiClientProvider = Provider<ApiClient>((ref) {
+  final authState = ref.watch(authStateProvider);
+  return ApiClient(token: authState.user?.token);
+});
+
+class ApiClient {
+  late final Dio _dio;
+
+  ApiClient({String? token}) {
+    _dio = Dio(BaseOptions(
+      baseUrl: 'http://localhost:8080/api/v1',
+      connectTimeout: const Duration(seconds: 10),
+      receiveTimeout: const Duration(seconds: 30),
+      headers: {
+        'Content-Type': 'application/json',
+        if (token != null) 'Authorization': 'Bearer $token',
+      },
+    ));
+  }
+
+  Future<Response> get(String path, {Map<String, dynamic>? params}) =>
+      _dio.get(path, queryParameters: params);
+
+  Future<Response> post(String path, {dynamic data}) =>
+      _dio.post(path, data: data);
+
+  Future<Response> put(String path, {dynamic data}) =>
+      _dio.put(path, data: data);
+
+  Future<Response> delete(String path) => _dio.delete(path);
+}