feat(CourseCard): 添加已购课程标识

在课程卡片上添加已购标识，当课程已被购买时显示

 <!--
  * @Date: 2025-03-20 20:36:36
  * @LastEditors: hookehuyr hookehuyr@gmail.com
- * @LastEditTime: 2025-09-24 22:59:47
+ * @LastEditTime: 2025-09-30 13:49:35
  * @FilePath: /mlaj/src/components/ui/CourseCard.vue
  * @Description: 文件描述
 -->
 <template>
   <router-link :to="linkTo || `/courses/${course.id}`" class="flex bg-white rounded-lg overflow-hidden shadow-sm">
-    <div class="w-1/3 h-28">
+    <div class="w-1/3 h-28 relative">
       <img
         :src="course.cover || 'https://cdn.ipadbiz.cn/mlaj/images/default_block.png'"
         :alt="course.title"
         class="w-full h-full object-cover"
       />
+      <!-- 已购标识 -->
+      <div
+        v-if="course.is_buy"
+        class="absolute top-0 left-0 bg-orange-500 text-white text-xs px-2 py-1 rounded-br-lg font-medium"
+        style="background-color: rgba(249, 115, 22, 0.85)"
+      >
+        已购
+      </div>
     </div>
     <div class="flex-1 p-3 flex flex-col justify-between">
       <div>