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>

frontend/src/views/UserManagement.vue

@@ -82,10 +82,7 @@
     <div class="user-list">
       <div v-for="u in filteredUsers" :key="u._id || u.id" class="user-card">
         <div class="user-info">
-          <div class="user-name">
-            {{ u.display_name || u.username }}
-            <span class="user-username" v-if="u.display_name">@{{ u.username }}</span>
-          </div>
+          <div class="user-name">{{ u.username }}</div>
           <div class="user-meta">
             <span class="user-role-badge" :class="'role-' + u.role">{{ roleLabel(u.role) }}</span>
             <span v-if="u.business_verified" class="biz-badge">💼 商业认证</span>