feat(stocking): 取消预估退税自动计算，改为纯手动输入

移除 watch 中的 (官方价-优惠)*11.5% 自动写入与手动编辑跟踪
(manualTaxRefundRows/prevTaxRefundValues/calcTaxRefund)，
保留手动输入后利润联动重算；placeholder 改为请输入。

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

src/views/crmpc/stocking/Add.vue

@@ -457,7 +457,7 @@
               <ElFormItem label="预估退税额" label-width="85px">
                 <ElInput
                   v-model="item.tax_refund"
-                  placeholder="正在计算预估退税额"
+                  placeholder="请输入预估退税额"
                   @input="handleUserEdit(idx)"
                 >
                   <template #suffix>

src/views/crmpc/stocking/final_add.vue

@@ -480,7 +480,7 @@
               <ElFormItem label="预估退税额" label-width="85px">
                 <ElInput 
                   v-model="item.tax_refund" 
-                  placeholder="正在计算预估退税额"
+                  placeholder="请输入预估退税额"
                   @input="handleUserEdit(idx)"
                 >
                   <template #suffix>

src/views/crmpc/stocking/stocking_edit.vue

@@ -479,7 +479,7 @@
               <ElFormItem label="预估退税额" label-width="85px">
                 <ElInput 
                   v-model="item.tax_refund" 
-                  placeholder="正在计算预估退税额"
+                  placeholder="请输入预估退税额"
                   @input="handleUserEdit(idx)"
                 >
                   <template #suffix>