fix: 首页滚动条防闪烁与颜色标注修正

- v-if 改为容器始终渲染 + min-height，避免数据加载后布局跳动
- 颜色标注改为"学习报告"/"学习残片"，准确描述元素类型

src/components/Welcome.vue

@@ -89,11 +89,15 @@ onMounted(() => {
 
 <template>
   <section class="welcome-page">
-    <div class="review-scroll-section surface-card" v-if="sessionGroups.length > 0">
-      <div class="review-scroll-header">
+    <div
+      class="review-scroll-section surface-card"
+      :class="{ 'is-empty': sessionGroups.length === 0 }"
+    >
+      <template v-if="sessionGroups.length > 0">
+        <div class="review-scroll-header">
         <span class="color-legend">
-          <span class="dot dot-report"></span> 有学习报告
-          <span class="dot dot-fragment"></span> 仅有学习残片
+          <span class="dot dot-report"></span> 学习报告
+          <span class="dot dot-fragment"></span> 学习残片
         </span>
       </div>
       <div class="review-scroll-track">
@@ -115,6 +119,7 @@ onMounted(() => {
           </span>
         </div>
       </div>
+      </template>
     </div>
 
     <div class="welcome-banner surface-card">
@@ -199,6 +204,11 @@ onMounted(() => {
 .review-scroll-section {
   padding: 14px 22px;
   overflow: hidden;
+  min-height: 90px;
+}
+
+.review-scroll-section.is-empty {
+  visibility: hidden;
 }
 
 .review-scroll-header {