fix(frontend): product/discover pages use API_BASE_URL for uploaded cover images

Convert relative coverImage paths to absolute URLs via config.API_BASE_URL in product detail and discover pages.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

cfc-frontend/pages/discover-detail/product-detail/product-detail.vue

@@ -6,7 +6,7 @@
     <scroll-view v-else-if="product.id" scroll-y class="content">
       <image
         class="cover"
-        :src="product.coverImage || '/static/default-product.png'"
+        :src="coverImageUrl || '/static/default-product.png'"
         mode="widthFix"
         @error="onImageError"
       />

cfc-frontend/pages/discover/index.vue

@@ -37,7 +37,7 @@
           class="product-card"
           @click="goDetail(item.id)"
         >
-          <image class="product-cover" :src="item.coverImage || '/static/default-product.png'" mode="aspectFill" />
+          <image class="product-cover" :src="getImageUrl(item.coverImage) || '/static/default-product.png'" mode="aspectFill" />
           <view class="product-info">
             <text class="product-name">{{ item.name }}</text>
             <view class="product-price-row">
@@ -127,6 +127,7 @@
 import PageBanner from '../../components/PageBanner.vue'
 import { productList, getFeaturedArticles, getMyMembership } from '@/utils/api.js'
 import WuxingSandbox from '../../components/wuxing-sandbox.vue'
+import config from '@/config.js'
 export default {
   components: { PageBanner, WuxingSandbox },
   data() {
@@ -218,6 +219,9 @@ export default {
     goArticleDetail(id) {
       uni.navigateTo({ url: '/pages/article-center/article-detail?id=' + id })
     },
+    getImageUrl: function(path) {
+      return config.API_BASE_URL + path
+    },
     goKnowledgeCenter() {
       uni.navigateTo({ url: '/pages/mind-extra/articles' })
     }