feat(client): 入库单价允许留空/0（待定价），移除 >0 校验

- 删除新建入库商品明细单价的「不能为空 + >0」校验，留空或填 0 视为「待定价」
- 待定价明细经审核后在入库单详情「确认进价」补填真实进价
- 清理未使用的 productCode 局部变量与 _productCodeOf 辅助方法

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

client/lib/screens/stock_in/stock_in_form_screen.dart

@@ -868,11 +868,8 @@ class _StockInFormScreenState extends ConsumerState<StockInFormScreen> {
         FilteringTextInputFormatter.allow(RegExp(r'^\d+\.?\d{0,2}'))
       ],
       onChanged: (_) => setState(() {}),
-      validator: (v) {
-        if (v == null || v.isEmpty) return '不能为空';
-        if ((double.tryParse(v) ?? 0) <= 0) return '>0';
-        return null;
-      },
+      // 单价允许为空或 0：表示「待定价」，入库审核后可在单据详情「确认进价」补填真实进价
+      validator: (_) => null,
     );
   }
 
@@ -976,15 +973,6 @@ class _StockInFormScreenState extends ConsumerState<StockInFormScreen> {
     );
   }
 
-  String _productCodeOf(_ItemRow item) =>
-      ref
-          .read(productNameListProvider)
-          .valueOrNull
-          ?.where((o) => o.id == item.selectedNameId)
-          .firstOrNull
-          ?.code ??
-      '';
-
   // ── 桌面表格：表头行 ──────────────────────────────────────────────────────
   Widget _buildItemTableHeader() {
     Widget th(String s) => Padding(
@@ -1019,7 +1007,6 @@ class _StockInFormScreenState extends ConsumerState<StockInFormScreen> {
     final qty = double.tryParse(item.qtyCtrl.text) ?? 0;
     final price = double.tryParse(item.priceCtrl.text) ?? 0;
     final amount = qty * price;
-    final productCode = _productCodeOf(item);
 
     Widget tc(String s, {Color? color, FontWeight? weight}) => Padding(
           padding: const EdgeInsets.symmetric(horizontal: 8, vertical: 12),
@@ -1166,7 +1153,6 @@ class _StockInFormScreenState extends ConsumerState<StockInFormScreen> {
     final qty = double.tryParse(item.qtyCtrl.text) ?? 0;
     final price = double.tryParse(item.priceCtrl.text) ?? 0;
     final amount = qty * price;
-    final productCode = _productCodeOf(item);
 
     return MobileListCard(
       title: Text('商品 ${index + 1}'),