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/core/utils/export_util.dart

@@ -0,0 +1,2 @@
+export 'export_util_stub.dart'
+    if (dart.library.js_interop) 'export_util_web.dart';

client/lib/core/utils/export_util_stub.dart

@@ -0,0 +1,33 @@
+import 'dart:io';
+import 'package:excel/excel.dart';
+import 'package:path_provider/path_provider.dart';
+import 'package:url_launcher/url_launcher.dart';
+
+void exportExcel({
+  required String filename,
+  required List<String> headers,
+  required List<List<dynamic>> rows,
+}) async {
+  final excel = Excel.createExcel();
+  final sheet = excel['Sheet1'];
+
+  sheet.appendRow(headers.map((h) => TextCellValue(h)).toList());
+
+  for (final row in rows) {
+    final cells = row.map<CellValue?>((v) {
+      if (v == null) return TextCellValue('');
+      if (v is double) return DoubleCellValue(v);
+      if (v is int) return IntCellValue(v);
+      return TextCellValue(v.toString());
+    }).toList();
+    sheet.appendRow(cells);
+  }
+
+  final bytes = excel.encode();
+  if (bytes == null) return;
+
+  final dir = await getDownloadsDirectory() ?? await getTemporaryDirectory();
+  final file = File('${dir.path}/$filename.xlsx');
+  await file.writeAsBytes(bytes);
+  await launchUrl(Uri.file(file.path));
+}

client/lib/core/utils/export_util_web.dart

@@ -0,0 +1,42 @@
+import 'dart:typed_data';
+import 'dart:js_interop';
+import 'package:excel/excel.dart';
+import 'package:web/web.dart' as web;
+
+void exportExcel({
+  required String filename,
+  required List<String> headers,
+  required List<List<dynamic>> rows,
+}) {
+  final excel = Excel.createExcel();
+  final sheet = excel['Sheet1'];
+
+  sheet.appendRow(headers.map((h) => TextCellValue(h)).toList());
+
+  for (final row in rows) {
+    final cells = row.map<CellValue?>((v) {
+      if (v == null) return TextCellValue('');
+      if (v is double) return DoubleCellValue(v);
+      if (v is int) return IntCellValue(v);
+      return TextCellValue(v.toString());
+    }).toList();
+    sheet.appendRow(cells);
+  }
+
+  final bytes = excel.encode();
+  if (bytes == null) return;
+
+  final uint8Array = Uint8List.fromList(bytes).toJS;
+  final blob = web.Blob(
+    <JSAny>[uint8Array].toJS,
+    web.BlobPropertyBag(
+        type: 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet'),
+  );
+  final url = web.URL.createObjectURL(blob);
+  final anchor = web.document.createElement('a') as web.HTMLAnchorElement
+    ..href = url
+    ..download = '$filename.xlsx'
+    ..click();
+  web.URL.revokeObjectURL(url);
+  anchor.remove();
+}