feat: 商品详情页、XLS导入修复、分页选择器、导出功能

后端：
- 新增 product_images 表，支持每商品最多5张图（服务端压缩至1200px/JPEG85%）
- products 表新增 public_id(UUID)、description 字段
- 新增商品详情接口、二维码接口、公开商品接口（无鉴权）
- 修复 XLS 导入：OLE2 magic bytes 检测 + 临时文件解析，兼容 extrame/xls
- 修复商品/名称/系列/规格三张表导入数据为0（LastCol()=0 bug）
- 所有导入接口返回 total/imported/skipped 统计
- config 新增 StorageConfig，支持 STORAGE_* 环境变量覆盖
- 种子数据修复：products 补 public_id、新增 product_images TRUNCATE、schema.sql 表名修正

前端：
- 商品详情页：图片上传/删除、描述内联编辑、二维码弹窗、公开链接复制
- 公开商品页：无鉴权路由 /product/:public_id，Flutter Web SPA
- 商品详情列表（批次追踪）商品名超链接跳转详情页
- 导航「商品管理」改名「商品详情」
- 所有列表表格新增每页条数选择（10/20/50/100）
- 表格列头内嵌筛选（FilterableColumnHeader）
- 导出 Excel 功能（入库/出库/库存/财务/批次/往来单位）
- 网络恢复自动刷新 + 离线缓存展示

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

client/lib/models/product_option.dart

@@ -0,0 +1,68 @@
+class ProductNameOption {
+  final int id;
+  final String? code;
+  final String name;
+  final String? remark;
+
+  const ProductNameOption({
+    required this.id,
+    this.code,
+    required this.name,
+    this.remark,
+  });
+
+  factory ProductNameOption.fromJson(Map<String, dynamic> json) =>
+      ProductNameOption(
+        id: (json['id'] as num).toInt(),
+        code: json['code'] as String?,
+        name: json['name'] as String,
+        remark: json['remark'] as String?,
+      );
+}
+
+class ProductSeriesOption {
+  final int id;
+  final String? code;
+  final String name;
+  final String? remark;
+
+  const ProductSeriesOption({
+    required this.id,
+    this.code,
+    required this.name,
+    this.remark,
+  });
+
+  factory ProductSeriesOption.fromJson(Map<String, dynamic> json) =>
+      ProductSeriesOption(
+        id: (json['id'] as num).toInt(),
+        code: json['code'] as String?,
+        name: json['name'] as String,
+        remark: json['remark'] as String?,
+      );
+}
+
+class ProductSpecOption {
+  final int id;
+  final String? code;
+  final String name;
+  final int quantity;
+  final String? remark;
+
+  const ProductSpecOption({
+    required this.id,
+    this.code,
+    required this.name,
+    this.quantity = 0,
+    this.remark,
+  });
+
+  factory ProductSpecOption.fromJson(Map<String, dynamic> json) =>
+      ProductSpecOption(
+        id: (json['id'] as num).toInt(),
+        code: json['code'] as String?,
+        name: json['name'] as String,
+        quantity: (json['quantity'] as num?)?.toInt() ?? 0,
+        remark: json['remark'] as String?,
+      );
+}