fix: username_changed字段加入认证查询，改名后铅笔和通知按钮正确隐藏

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

frontend/src/components/UserMenu.vue

@@ -41,8 +41,9 @@
               <div v-if="!n.is_read" class="notif-actions">
                 <!-- 搜索未收录通知：已添加按钮 -->
                 <button v-if="isSearchMissing(n)" class="notif-action-btn notif-btn-added" @click="markAdded(n)">已添加</button>
-                <!-- 用户名更新通知：去修改按钮 -->
-                <button v-else-if="isUsernameNotice(n)" class="notif-action-btn notif-btn-plan" @click="goRename(n)">去修改</button>
+                <!-- 用户名更新通知：去修改按钮（已改过则显示已读） -->
+                <button v-else-if="isUsernameNotice(n) && !auth.user.username_changed" class="notif-action-btn notif-btn-plan" @click="goRename(n)">去修改</button>
+                <button v-else-if="isUsernameNotice(n) && auth.user.username_changed" class="notif-mark-one" @click="markOneRead(n)">已读</button>
                 <!-- 方案请求通知：去定制按钮 -->
                 <button v-else-if="isPlanRequest(n)" class="notif-action-btn notif-btn-plan" @click="goPlanDesign(n)">去定制</button>
                 <!-- 审核类通知：去审核按钮 -->