fix: CartService 回填 skuId + wisdom 阅读统计条件优化

- CartService.toCartItemDTO(): setSkuId(cart.getSkuId()) 补回
- wisdom/index.vue: 阅读时长条件从 !== undefined 改为 > 0

cfc-backend/src/main/java/com/etotem/cfc/service/CartService.java

@@ -133,6 +133,7 @@ public class CartService {
         dto.setId(cart.getId());
         dto.setUserId(cart.getUserId());
         dto.setProductId(cart.getProductId());
+        dto.setSkuId(cart.getSkuId());
         dto.setQuantity(cart.getQuantity());
         if (product != null) {
             dto.setProductName(product.getName());

cfc-frontend/pages/wisdom/index.vue

@@ -49,7 +49,7 @@
               <text class="rsc-title">📚 本月阅读</text>
             </view>
             <view class="rsc-body">
-              <text class="rsc-stat-value" v-if="readingStats && readingStats.totalDurationSeconds !== undefined">
+              <text class="rsc-stat-value" v-if="readingStats && readingStats.totalDurationSeconds > 0">
                 {{ (readingStats.totalDurationSeconds / 3600).toFixed(1) }} 小时
               </text>
               <text class="rsc-stat-value" v-else-if="readingStats">暂无阅读记录</text>