fix: 首页滚动条始终渲染 DOM，消除加载生硬感

去掉 v-if 和 visibility 条件控制，滚动区域结构在页面加载时即存在于 DOM 中，
芯片通过 v-for 响应式插入，避免数据到达时整块 DOM 节点创建/销毁导致的跳变

src/components/Welcome.vue

@@ -89,12 +89,8 @@ onMounted(() => {
 
 <template>
   <section class="welcome-page">
-    <div
-      class="review-scroll-section surface-card"
-      :class="{ 'is-empty': sessionGroups.length === 0 }"
-    >
-      <template v-if="sessionGroups.length > 0">
-        <div class="review-scroll-header">
+    <div class="review-scroll-section surface-card">
+      <div class="review-scroll-header">
         <span class="color-legend">
           <span class="dot dot-report"></span> 学习报告
           <span class="dot dot-fragment"></span> 学习残片
@@ -119,7 +115,6 @@ onMounted(() => {
           </span>
         </div>
       </div>
-      </template>
     </div>
 
     <div class="welcome-banner surface-card">
@@ -204,11 +199,6 @@ onMounted(() => {
 .review-scroll-section {
   padding: 14px 22px;
   overflow: hidden;
-  min-height: 90px;
-}
-
-.review-scroll-section.is-empty {
-  visibility: hidden;
 }
 
 .review-scroll-header {