feat(client): 二维码打印功能

在商品详情二维码弹窗中新增「打印」按钮，使用 printing + pdf
生成含商品名称、编号、规格和二维码图片的 PDF，触发浏览器打印对话框。
嵌入 NotoSansSC-Regular 字体保证中文正常显示。

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

client/lib/screens/products/product_detail_screen.dart

@@ -4,6 +4,9 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:pdf/pdf.dart';
+import 'package:pdf/widgets.dart' as pw;
+import 'package:printing/printing.dart';
 import '../../core/config/app_config.dart';
 import '../../core/theme/app_theme.dart';
 import '../../models/product.dart';
@@ -152,7 +155,7 @@ class _ProductDetailScreenState extends ConsumerState<ProductDetailScreen> {
   void _showQRCode() {
     showDialog(
       context: context,
-      builder: (_) => _QRCodeDialog(productId: widget.productId),
+      builder: (_) => _QRCodeDialog(productId: widget.productId, product: _product!),
     );
   }
 
@@ -498,7 +501,8 @@ class _UploadButton extends StatelessWidget {
 
 class _QRCodeDialog extends ConsumerStatefulWidget {
   final int productId;
-  const _QRCodeDialog({required this.productId});
+  final Product product;
+  const _QRCodeDialog({required this.productId, required this.product});
 
   @override
   ConsumerState<_QRCodeDialog> createState() => _QRCodeDialogState();
@@ -506,13 +510,51 @@ class _QRCodeDialog extends ConsumerStatefulWidget {
 
 class _QRCodeDialogState extends ConsumerState<_QRCodeDialog> {
   late Future<Uint8List> _future;
+  Uint8List? _bytes;
+  bool _printing = false;
 
   @override
   void initState() {
     super.initState();
     _future = ref
         .read(productRepositoryProvider)
-        .getQRCodeBytes(widget.productId);
+        .getQRCodeBytes(widget.productId)
+      ..then((b) => _bytes = b);
+  }
+
+  Future<void> _print() async {
+    if (_bytes == null || _printing) return;
+    setState(() => _printing = true);
+    try {
+      final fontData = await rootBundle.load('assets/fonts/NotoSansSC-Regular.ttf');
+      final font = pw.Font.ttf(fontData);
+      final p = widget.product;
+      final doc = pw.Document();
+      final qrImage = pw.MemoryImage(_bytes!);
+      doc.addPage(pw.Page(
+        pageFormat: PdfPageFormat.a4,
+        build: (_) => pw.Center(
+          child: pw.Column(
+            mainAxisSize: pw.MainAxisSize.min,
+            children: [
+              pw.Image(qrImage, width: 200, height: 200),
+              pw.SizedBox(height: 12),
+              pw.Text(p.name,
+                  style: pw.TextStyle(font: font, fontSize: 18, fontWeight: pw.FontWeight.bold)),
+              pw.SizedBox(height: 4),
+              pw.Text('编号：${p.code}',
+                  style: pw.TextStyle(font: font, fontSize: 13)),
+              if ((p.spec ?? '').isNotEmpty)
+                pw.Text('规格：${p.spec}',
+                    style: pw.TextStyle(font: font, fontSize: 13)),
+            ],
+          ),
+        ),
+      ));
+      await Printing.layoutPdf(onLayout: (_) async => doc.save());
+    } finally {
+      if (mounted) setState(() => _printing = false);
+    }
   }
 
   @override
@@ -520,8 +562,8 @@ class _QRCodeDialogState extends ConsumerState<_QRCodeDialog> {
     return AlertDialog(
       title: const Text('商品二维码'),
       content: SizedBox(
-        width: 200,
-        height: 200,
+        width: 220,
+        height: 220,
         child: FutureBuilder<Uint8List>(
           future: _future,
           builder: (_, snap) {
@@ -540,6 +582,15 @@ class _QRCodeDialogState extends ConsumerState<_QRCodeDialog> {
           onPressed: () => Navigator.of(context).pop(),
           child: const Text('关闭'),
         ),
+        FilledButton.icon(
+          onPressed: _bytes == null ? null : (_printing ? null : _print),
+          icon: _printing
+              ? const SizedBox(
+                  width: 14, height: 14,
+                  child: CircularProgressIndicator(strokeWidth: 2, color: Colors.white))
+              : const Icon(Icons.print, size: 16),
+          label: const Text('打印'),
+        ),
       ],
     );
   }