style(choiceQuestionDetailDialog): 调整选择题选项网格布局为3列

将原4列的网格布局改为3列，优化选项展示排版

src/views/Student/components/choiceQuestionDetailDialog.vue

@@ -3033,7 +3033,7 @@ onUnmounted(() => {
         width: 100%;
         height: auto;
         display: grid;
-        grid-template-columns: repeat(4, 1fr);
+        grid-template-columns: repeat(3, 1fr);
         gap: 20px;
         margin-top: 40px;