fix: 去掉账号设置里显示名称模块，用户管理去掉重复@username

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/views/MyDiary.vue

@@ -211,12 +211,6 @@
       <div class="section-card">
         <h4>👤 账号设置</h4>
 
-        <div class="form-group">
-          <label>显示名称</label>
-          <input v-model="displayName" class="form-input" />
-          <button class="btn-primary btn-sm" style="margin-top:6px" @click="updateDisplayName">保存</button>
-        </div>
-
         <div class="form-group">
           <label>用户名</label>
           <div class="form-static">{{ auth.user.username }}</div>