feat(client): 打印任务默认名 类型_时间戳（标签/入库单/出库单）

Printing.layoutPdf 传 name：标签_yyyyMMdd_HHmmss / 入库单_... / 出库单_...，
作为打印任务名与「打印到 PDF」的默认文件名。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/core/utils/print_util_stub.dart

@@ -17,6 +17,13 @@ Future<pw.Font> _loadBoldFont() async {
   return pw.Font.ttf(data);
 }
 
+/// 打印任务/另存默认文件名的时间戳：yyyyMMdd_HHmmss
+String _fileTs() {
+  final n = DateTime.now();
+  String p(int v) => v.toString().padLeft(2, '0');
+  return '${n.year}${p(n.month)}${p(n.day)}_${p(n.hour)}${p(n.minute)}${p(n.second)}';
+}
+
 // ── 设计色彩 token ──────────────────────────────────────────────────────────
 const _navy     = PdfColor(0.122, 0.165, 0.227);  // #1F2A3A  header/chip
 const _cream    = PdfColor(0.957, 0.925, 0.847);  // #F4ECD8  footer/reversed text
@@ -191,7 +198,8 @@ Future<void> printProductLabelImpl({
     ),
   ));
 
-    await Printing.layoutPdf(onLayout: (_) async => doc.save());
+    await Printing.layoutPdf(
+        name: '标签_${_fileTs()}', onLayout: (_) async => doc.save());
   } catch (e, st) {
     reportError(e, st);
     throw Exception('打印失败，请检查打印机连接和驱动是否正常。\n详情：$e');
@@ -431,7 +439,8 @@ Future<void> printStockInOrderImpl(StockInOrder order) async {
       ),
     ],
   ));
-    await Printing.layoutPdf(onLayout: (_) async => doc.save());
+    await Printing.layoutPdf(
+        name: '入库单_${_fileTs()}', onLayout: (_) async => doc.save());
   } catch (e, st) {
     reportError(e, st);
     throw Exception('打印失败，请检查打印机连接和驱动是否正常。\n详情：$e');
@@ -486,7 +495,8 @@ Future<void> printStockOutOrderImpl(StockOutOrder order) async {
     ],
   ));
   try {
-    await Printing.layoutPdf(onLayout: (_) async => doc.save());
+    await Printing.layoutPdf(
+        name: '出库单_${_fileTs()}', onLayout: (_) async => doc.save());
   } catch (e, st) {
     reportError(e, st);
     throw Exception('打印失败，请检查打印机连接和驱动是否正常。\n详情：$e');