style(student/choiceQuestionDetail): 替换硬编码S为动态首字母

将多处固定显示的<span>S</span>改为根据item.name动态获取首字母，统一列表项的前缀展示逻辑

src/views/Student/components/choiceQuestionDetailDialog.vue

@@ -244,7 +244,7 @@
         <div class="c_t15_content" v-show="!lookWorkData">
           <div class="c_t15_c_item" v-for="item in processedWorkArray" :key="item.id" @click.stop="lookWork(item.id)">
             <div class="c_t15_c_i_top">
-              <span>S</span>
+              <span>{{ item.name.charAt(0) }}</span>
               <div>{{ item.name }}</div>
             </div>
             <div class="c_t15_c_i_bottom">
@@ -345,7 +345,7 @@
           <div class="c_t15_c_item" v-for="item in processedWorkArray" :key="item.id"
             @click.stop="item.content && item.content.fileList.length > 0 ? lookWork(item.id) : ''">
             <div class="c_t15_c_i_top">
-              <span>S</span>
+              <span>{{ item.name.charAt(0) }}</span>
               <div>{{ item.name }}</div>
             </div>
             <div class="c_t73_c_i_bottom" v-if="item.content && item.content.fileList.length > 0">
@@ -427,7 +427,7 @@
         <div class="c_t72_content" v-show="!lookWorkData">
           <div class="c_t72_c_item" v-for="item in processedWorkArray" :key="item.id" @click.stop="lookWork(item.id)">
             <div class="c_t72_c_i_top">
-              <span>S</span>
+              <span>{{ item.name.charAt(0) }}</span>
               <div>{{ item.name }}</div>
             </div>
             <div class="bottom_btn" v-if="likeValue">
@@ -547,7 +547,7 @@
         <div class="c_t73_content" v-show="!lookWorkData">
           <div class="c_t73_c_item" v-for="item in processedWorkArray" :key="item.id" @click.stop="lookWork(item.id)">
             <div class="c_t73_c_i_top">
-              <span>S</span>
+              <span>{{ item.name.charAt(0) }}</span>
               <div>{{ item.name }}</div>
             </div>
             <div class="c_t73_c_i_bottom">