refactor(课程详情页): 优化课程详情页的标签显示逻辑

根据课程内容动态显示标签页，移除默认值'N/A'

@@ -57,20 +57,14 @@
           <!-- Tab Content -->
           <div class="p-4">
             <div v-if="activeTab === '课程特色'">
-              <!-- <ul class="list-disc pl-5 space-y-2 text-gray-700">
-                <li>小班授课，更多关注</li>
-                <li>名师授课，经验丰富</li>
-                <li>随堂练习，巩固知识</li>
-                <li>及时反馈，调整教学</li>
-              </ul> -->
-              <div v-html="course?.feature || 'N/A'"></div>
+              <div v-html="course?.feature"></div>
             </div>
 
             <div v-if="activeTab === '课程大纲'">
               <div class="space-y-4">
                 <div v-for="(item, index) in displayedSchedule" :key="index" class="border-l-2 border-green-500 pl-3">
                   <h4 class="font-medium text-gray-800">{{ item.title }}</h4>
-                  <p class="text-sm text-gray-600 mt-1">{{ item.duration }}分钟 · {{ item.schedule_time || 'N/A' }}个小节</p>
+                  <p class="text-sm text-gray-600 mt-1">{{ item.duration }}分钟 · {{ item.schedule_time }}个小节</p>
                 </div>
                 <div v-if="course?.schedule?.length > 4" class="flex justify-center mt-4">
                   <button @click="toggleSchedule"
@@ -84,13 +78,13 @@
 
             <div v-if="activeTab === '课程亮点'">
               <div class="space-y-3 text-gray-700">
-                <div v-html="course?.highlights || 'N/A'"></div>
+                <div v-html="course?.highlights"></div>
               </div>
             </div>
 
             <div v-if="activeTab === '学习目标'">
               <div class="space-y-3 text-gray-700">
-                <div v-html="course?.learning_goal || 'N/A'"></div>
+                <div v-html="course?.learning_goal"></div>
               </div>
             </div>
           </div>
@@ -273,12 +267,16 @@ const toggleFavorite = async () => {
 }
 
 // Curriculum items
-const curriculumItems = [
-  { title: '课程特色', active: true },
-  { title: '课程大纲', active: false },
-  { title: '课程亮点', active: false },
-  { title: '学习目标', active: false },
-]
+const curriculumItems = computed(() => {
+  if (!course.value) return [];
+  
+  return [
+    { title: '课程特色', active: activeTab.value === '课程特色', show: !!course.value.feature },
+    { title: '课程大纲', active: activeTab.value === '课程大纲', show: !!(course.value.schedule && course.value.schedule.length > 0) },
+    { title: '课程亮点', active: activeTab.value === '课程亮点', show: !!course.value.highlights },
+    { title: '学习目标', active: activeTab.value === '学习目标', show: !!course.value.learning_goal },
+  ].filter(item => item.show);
+});
 
 // Handle image error
 const handleImageError = (e) => {
@@ -370,6 +368,13 @@ onMounted(async () => {
       isFavorite.value = foundCourse.is_favorite;
       isPurchased.value = foundCourse.is_buy;
       isReviewed.value = foundCourse.is_comment;
+      
+      // 设置默认选中的 tab，确保选中的 tab 有内容
+      const availableTabs = curriculumItems.value;
+      if (availableTabs.length > 0 && !availableTabs.some(item => item.title === activeTab.value)) {
+        activeTab.value = availableTabs[0].title;
+      }
+      
       // 获取评论列表
       await fetchCommentList()
     } else {