fix(client): 修复标签打印预览三个问题

- 打印完成后自动关闭预览弹窗
- 打印机列表改为仅显示热敏打印机，无热敏打印机时显示"无打印机可用"并禁用打印按钮
- 库存列表表格行"打标签"按钮改为调用预览弹窗（之前仍走旧直接打印逻辑）

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

client/lib/widgets/label_preview_dialog.dart

@@ -36,6 +36,7 @@ class _LabelPreviewDialogState extends State<LabelPreviewDialog> {
   List<String> _printers = [];
   String? _printer;
   bool _printing = false;
+  bool _printersChecked = false;
   String _status = '';
 
   @override
@@ -63,6 +64,7 @@ class _LabelPreviewDialogState extends State<LabelPreviewDialog> {
       setState(() {
         _printers = printers;
         _printer = selected;
+        _printersChecked = true;
       });
     }
 
@@ -147,6 +149,7 @@ class _LabelPreviewDialogState extends State<LabelPreviewDialog> {
         _printing = false;
         _status = '完成，共打印 $done 张';
       });
+      Navigator.of(context).pop();
     }
   }
 
@@ -329,8 +332,9 @@ class _LabelPreviewDialogState extends State<LabelPreviewDialog> {
                   const SizedBox(width: 8),
                   Expanded(
                     child: _printers.isEmpty
-                        ? const Text('检测中或未检测到打印机...',
-                            style: TextStyle(
+                        ? Text(
+                            _printersChecked ? '无打印机可用' : '正在检测打印机...',
+                            style: const TextStyle(
                                 fontSize: 13,
                                 color: AppTheme.textSecondary))
                         : DropdownButtonHideUnderline(
@@ -419,7 +423,9 @@ class _LabelPreviewDialogState extends State<LabelPreviewDialog> {
                   ),
                   const SizedBox(width: 8),
                   FilledButton(
-                    onPressed: (_printing || totalCount == 0) ? null : _doPrint,
+                    onPressed: (_printing || totalCount == 0 || _printer == null)
+                        ? null
+                        : _doPrint,
                     child: Text(
                         _printing ? '打印中...' : '打印 $totalCount 张'),
                   ),