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/widgets/data_table_card.dart

@@ -9,6 +9,7 @@ class DataTableCard extends StatefulWidget {
   final int page;
   final int pageSize;
   final ValueChanged<int>? onPageChanged;
+  final ValueChanged<int>? onPageSizeChanged;
 
   const DataTableCard({
     super.key,
@@ -19,6 +20,7 @@ class DataTableCard extends StatefulWidget {
     this.page = 1,
     this.pageSize = 20,
     this.onPageChanged,
+    this.onPageSizeChanged,
   });
 
   @override
@@ -91,6 +93,7 @@ class _DataTableCardState extends State<DataTableCard> {
                   total: widget.totalCount!,
                   pageSize: widget.pageSize,
                   onPageChanged: widget.onPageChanged,
+                  onPageSizeChanged: widget.onPageSizeChanged,
                 ),
               ],
             ),
@@ -157,19 +160,45 @@ class _PaginationBar extends StatelessWidget {
   final int total;
   final int pageSize;
   final ValueChanged<int>? onPageChanged;
+  final ValueChanged<int>? onPageSizeChanged;
+
+  static const _pageSizeOptions = [10, 20, 50, 100];
 
   const _PaginationBar({
     required this.page,
     required this.total,
     required this.pageSize,
     this.onPageChanged,
+    this.onPageSizeChanged,
   });
 
   @override
   Widget build(BuildContext context) {
     final totalPages = (total / pageSize).ceil().clamp(1, 9999);
+    // 确保当前 pageSize 在选项中（如默认值 50 不在列表时仍能显示）
+    final effectiveSize = _pageSizeOptions.contains(pageSize) ? pageSize : _pageSizeOptions.first;
     return Row(
       children: [
+        // 每页条数选择器
+        if (onPageSizeChanged != null) ...[
+          const Text('每页', style: TextStyle(fontSize: 13, color: AppTheme.textSecondary)),
+          const SizedBox(width: 4),
+          DropdownButtonHideUnderline(
+            child: DropdownButton<int>(
+              value: effectiveSize,
+              isDense: true,
+              style: const TextStyle(fontSize: 13, color: AppTheme.textPrimary),
+              items: _pageSizeOptions
+                  .map((n) => DropdownMenuItem(value: n, child: Text('$n 条')))
+                  .toList(),
+              onChanged: (v) {
+                if (v != null) onPageSizeChanged!(v);
+              },
+            ),
+          ),
+          const SizedBox(width: 16),
+        ],
+        // 翻页控件
         IconButton(
           icon: const Icon(Icons.first_page, size: 18),
           onPressed: page > 1 ? () => onPageChanged?.call(1) : null,
@@ -182,20 +211,16 @@ class _PaginationBar extends StatelessWidget {
           padding: EdgeInsets.zero,
           constraints: const BoxConstraints(minWidth: 28, minHeight: 28),
         ),
-        Text('$page / $totalPages',
-            style: const TextStyle(fontSize: 13)),
+        Text('$page / $totalPages', style: const TextStyle(fontSize: 13)),
         IconButton(
           icon: const Icon(Icons.chevron_right, size: 18),
-          onPressed:
-              page < totalPages ? () => onPageChanged?.call(page + 1) : null,
+          onPressed: page < totalPages ? () => onPageChanged?.call(page + 1) : null,
           padding: EdgeInsets.zero,
           constraints: const BoxConstraints(minWidth: 28, minHeight: 28),
         ),
         IconButton(
           icon: const Icon(Icons.last_page, size: 18),
-          onPressed: page < totalPages
-              ? () => onPageChanged?.call(totalPages)
-              : null,
+          onPressed: page < totalPages ? () => onPageChanged?.call(totalPages) : null,
           padding: EdgeInsets.zero,
           constraints: const BoxConstraints(minWidth: 28, minHeight: 28),
         ),