refactor: 题目描述单栏显示（恢复全宽编辑器）并移除实时预览

src/view/admin/ProblemManageView.vue

@@ -524,14 +524,8 @@ onMounted(() => {
           <el-input v-model="problemForm.title" placeholder="请输入题目标题" />
         </el-form-item>
         <el-form-item label="描述" prop="description">
-          <div class="markdown-split">
-            <div class="markdown-editor-wrapper">
-              <MdEditor :value="problemForm.description || ''" :plugins="mdPlugins" @change="(v: string) => (problemForm.description = v)" />
-            </div>
-            <div class="markdown-preview markdown-body">
-              <div v-if="!problemForm.description" class="preview-empty">暂无内容，输入后实时预览</div>
-              <MdViewer v-else :value="problemForm.description || ''" :plugins="mdPlugins" />
-            </div>
+          <div class="markdown-editor-wrapper">
+            <MdEditor :value="problemForm.description || ''" :plugins="mdPlugins" @change="(v: string) => (problemForm.description = v)" />
           </div>
         </el-form-item>
         <el-form-item label="时间限制" prop="timeLimitMs">
@@ -703,32 +697,6 @@ onMounted(() => {
   gap: 12px;
 }
 
-.markdown-split {
-  display: flex;
-  gap: 12px;
-  width: 100%;
-}
-
-.markdown-split .markdown-editor-wrapper,
-.markdown-split .markdown-preview {
-  flex: 1;
-  min-width: 0;
-}
-
-.markdown-preview {
-  height: 300px;
-  overflow: auto;
-  border: 1px solid var(--el-border-color);
-  border-radius: 4px;
-  padding: 12px;
-  background: var(--el-bg-color);
-}
-
-.preview-empty {
-  color: var(--el-text-color-placeholder);
-  font-size: 13px;
-}
-
 .markdown-editor-wrapper :deep(.bytemd) {
   height: 300px;
 }