feat(client): 表格加横向滚动条 + 库存页列显隐

- DataTableCard 横向/纵向 ScrollView 各加 Scrollbar(thumbVisibility:true)，
  Windows 鼠标用户可拖动滚动条查看右侧截断的列（6 个表格全受益）
- 库存查询页新增「显示字段」按钮，可按需隐藏生产日期/供应商/备注等列，
  列变少后横向溢出大幅缓解；商品编码/名称/操作三列锁定不可隐藏

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

client/lib/widgets/data_table_card.dart

@@ -36,10 +36,14 @@ class DataTableCard extends StatefulWidget {
 class _DataTableCardState extends State<DataTableCard> {
   // 当前悬停行的索引（-1 表示无悬停）
   final _hoveredRow = ValueNotifier<int>(-1);
+  final _vCtrl = ScrollController();
+  final _hCtrl = ScrollController();
 
   @override
   void dispose() {
     _hoveredRow.dispose();
+    _vCtrl.dispose();
+    _hCtrl.dispose();
     super.dispose();
   }
 
@@ -89,24 +93,35 @@ class _DataTableCardState extends State<DataTableCard> {
 
   Widget _buildTable() {
     return LayoutBuilder(
-      builder: (context, constraints) => SingleChildScrollView(
+      builder: (context, constraints) => Scrollbar(
+        controller: _vCtrl,
+        thumbVisibility: true,
         child: SingleChildScrollView(
-          scrollDirection: Axis.horizontal,
-          child: ConstrainedBox(
-            constraints: BoxConstraints(minWidth: constraints.maxWidth),
-            child: Table(
-              defaultColumnWidth: const IntrinsicColumnWidth(),
-              border: TableBorder(
-                horizontalInside: BorderSide(
-                  color: Colors.grey.shade200,
-                  width: 0.5,
+          controller: _vCtrl,
+          child: Scrollbar(
+            controller: _hCtrl,
+            thumbVisibility: true,
+            scrollbarOrientation: ScrollbarOrientation.bottom,
+            child: SingleChildScrollView(
+              controller: _hCtrl,
+              scrollDirection: Axis.horizontal,
+              child: ConstrainedBox(
+                constraints: BoxConstraints(minWidth: constraints.maxWidth),
+                child: Table(
+                  defaultColumnWidth: const IntrinsicColumnWidth(),
+                  border: TableBorder(
+                    horizontalInside: BorderSide(
+                      color: Colors.grey.shade200,
+                      width: 0.5,
+                    ),
+                  ),
+                  children: [
+                    _buildHeaderRow(),
+                    for (int i = 0; i < widget.rows.length; i++)
+                      _buildDataRow(i, widget.rows[i]),
+                  ],
                 ),
               ),
-              children: [
-                _buildHeaderRow(),
-                for (int i = 0; i < widget.rows.length; i++)
-                  _buildDataRow(i, widget.rows[i]),
-              ],
             ),
           ),
         ),