fix(speaking): ready-footer 改成 column 方向让 session-error 显示在按钮下方

原来 display: flex 默认 row,错误 <p> 会贴在按钮右侧,和计划的
.session-error-text { margin: 8px 0 0; } 语义冲突。补 flex-direction: column
+ align-items: center。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/views/Editor/EnglishSpeaking/preview/TopicDiscussionPreview.vue

@@ -342,7 +342,8 @@ onUnmounted(() => { speakingStore.setPreviewState('ready') })
   border-top: 1px solid #f9fafb;
   background: #fafbfc;
   display: flex;
-  justify-content: center;
+  flex-direction: column;
+  align-items: center;
 }
 .start-btn {
   display: inline-flex;