fix: 添加购物车时检查购物资格

- 后端 CartHandler.Add 新增购物资格检查
- 后端新增库存检查和最大购买数量检查
- 前端商品详情页添加资格不足提示和视觉反馈
- 显示用户当前资格和所需资格总数

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/views/user/ProductDetail.vue

@@ -33,8 +33,15 @@
           <p class="price">¥{{ product.price }}</p>
           <p class="description">{{ product.description }}</p>
           <div class="meta">
-            <el-tag v-if="product.require_credit" type="warning">{{ $t('product.requireCredit') }}: {{ product.credit_cost }}</el-tag>
-            <el-tag v-if="product.credit_reward > 0" type="success">+{{ product.credit_reward }} {{ $t('product.creditReward') }}</el-tag>
+            <el-tag v-if="product.require_credit" :type="hasEnoughCredit() ? 'warning' : 'danger'">
+              {{ $t('product.requireCredit') }}: {{ product.credit_cost * quantity }}
+              <span v-if="!hasEnoughCredit()"> (资格不足)</span>
+            </el-tag>
+            <el-tag v-if="product.credit_reward > 0" type="success">+{{ product.credit_reward * quantity }} {{ $t('product.creditReward') }}</el-tag>
+          </div>
+          <div class="user-credits" v-if="product.require_credit">
+            <span class="credits-label">当前资格：</span>
+            <span class="credits-value" :class="{ 'insufficient': !hasEnoughCredit() }">{{ userCredits }}</span>
           </div>
           <div class="custom-fields" v-if="product.custom_fields?.length">
             <h4>{{ $t('product.customFields') }}</h4>
@@ -59,7 +66,7 @@ import { ref, computed, onMounted } from 'vue'
 import { useRoute, useRouter } from 'vue-router'
 import { ElMessage } from 'element-plus'
 import { Goods } from '@element-plus/icons-vue'
-import { productApi } from '../../api'
+import { productApi, userApi } from '../../api'
 import { useCartStore } from '../../store/cart'
 import { useI18n } from 'vue-i18n'
 import { getImageUrl } from '../../utils/image'
@@ -71,24 +78,62 @@ const { t } = useI18n()
 const cartStore = useCartStore()
 const product = ref<any>(null)
 const quantity = ref(1)
+const userCredits = ref(0)
 
 const productImages = computed(() => {
   if (!product.value?.images) return []
   return product.value.images.split(',').map((s: string) => s.trim()).filter(Boolean).map((url: string) => getImageUrl(url))
 })
 
+// 检查用户是否有足够资格购买
+function hasEnoughCredit(): boolean {
+  if (!product.value || !product.value.require_credit) return true
+  const totalCost = product.value.credit_cost * quantity.value
+  return userCredits.value >= totalCost
+}
+
+// 获取用户资格信息
+async function fetchUserCredits() {
+  try {
+    const res: any = await userApi.getProfile()
+    userCredits.value = res.data?.purchase_credits || 0
+  } catch {}
+}
+
 onMounted(async () => {
   const res: any = await productApi.getById(Number(route.params.id))
   product.value = res.data
+  // 设置默认数量为最小购买量
+  if (product.value?.min_purchase && product.value.min_purchase > 1) {
+    quantity.value = product.value.min_purchase
+  }
+  // 获取用户资格信息
+  await fetchUserCredits()
 })
 
 async function addToCart() {
-  await cartStore.addItem(product.value.id, quantity.value)
-  ElMessage.success(t('product.addToCart') + ' ✓')
+  // 检查购物资格
+  if (!hasEnoughCredit()) {
+    const totalCost = product.value.credit_cost * quantity.value
+    ElMessage.warning(`资格不足！需要 ${totalCost} 资格，您当前只有 ${userCredits.value} 资格`)
+    return
+  }
+
+  try {
+    await cartStore.addItem(product.value.id, quantity.value)
+    ElMessage.success(t('product.addToCart') + ' ✓')
+  } catch (err: any) {
+    // 处理后端返回的错误
+    const errorMsg = err.response?.data?.error || '添加购物车失败'
+    ElMessage.error(errorMsg)
+  }
 }
 
 function buyNow() {
-  addToCart().then(() => router.push('/cart'))
+  addToCart().then(() => {
+    // 只有成功添加后才跳转
+    router.push('/cart')
+  })
 }
 </script>
 
@@ -123,6 +168,30 @@ h1 { font-size: 28px; color: #fff; margin-bottom: 16px; }
 .price { font-size: 32px; font-weight: 700; color: #4e6ef2; margin-bottom: 16px; }
 .description { color: rgba(255, 255, 255, 0.5); margin-bottom: 16px; line-height: 1.6; }
 .meta { display: flex; gap: 8px; margin-bottom: 16px; }
+.user-credits {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  margin-bottom: 16px;
+  padding: 8px 12px;
+  background: rgba(255, 255, 255, 0.04);
+  border-radius: 6px;
+
+  .credits-label {
+    color: rgba(255, 255, 255, 0.6);
+    font-size: 14px;
+  }
+
+  .credits-value {
+    color: #10b981;
+    font-size: 16px;
+    font-weight: 600;
+
+    &.insufficient {
+      color: #f56c6c;
+    }
+  }
+}
 .custom-fields { margin-bottom: 24px; h4 { color: #fff; margin-bottom: 8px; } .field-item { color: rgba(255, 255, 255, 0.7); margin-bottom: 4px; .field-name { color: rgba(255, 255, 255, 0.5); margin-right: 8px; } } }
 .purchase-section { display: flex; gap: 12px; align-items: center; margin-top: 24px; }