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.dart

@@ -1,5 +1,8 @@
+import 'product_image.dart';
+
 class Product {
   final int id;
+  final String publicId;
   final String code;
   final String? barcode;
   final String name;
@@ -11,11 +14,14 @@ class Product {
   final double? purchasePrice;
   final double? salePrice;
   final int? minStock;
+  final String? description;
   final String? remark;
   final Map<String, dynamic>? customFields;
+  final List<ProductImage> images;
 
   const Product({
     required this.id,
+    this.publicId = '',
     required this.code,
     this.barcode,
     required this.name,
@@ -27,12 +33,15 @@ class Product {
     this.purchasePrice,
     this.salePrice,
     this.minStock,
+    this.description,
     this.remark,
     this.customFields,
+    this.images = const [],
   });
 
   factory Product.fromJson(Map<String, dynamic> json) => Product(
         id: (json['id'] as num).toInt(),
+        publicId: json['public_id'] as String? ?? '',
         code: json['code'] as String? ?? '',
         barcode: json['barcode'] as String?,
         name: json['name'] as String,
@@ -52,8 +61,13 @@ class Product {
         minStock: json['min_stock'] != null
             ? (json['min_stock'] as num).toInt()
             : null,
+        description: json['description'] as String?,
         remark: json['remark'] as String?,
         customFields: json['custom_fields'] as Map<String, dynamic>?,
+        images: (json['images'] as List<dynamic>?)
+                ?.map((e) => ProductImage.fromJson(e as Map<String, dynamic>))
+                .toList() ??
+            [],
       );
 
   Map<String, dynamic> toJson() => {
@@ -68,6 +82,7 @@ class Product {
         if (purchasePrice != null) 'purchase_price': purchasePrice,
         if (salePrice != null) 'sale_price': salePrice,
         if (minStock != null) 'min_stock': minStock,
+        'description': description ?? '',
         if (remark != null) 'remark': remark,
       };
 }