feat: 取消显示名称，统一用户名，大小写不敏感去重，一次性改名

- 注册: 去掉display_name字段，用户名大小写不敏感去重
- 登录: 大小写不敏感匹配
- 用户菜单: 显示用户名+改名按钮（只能改一次）
- /api/me/username: 一次性改名API
- 启动时: sync display_name=username, 发通知告知用户
- 前端: 所有display_name显示改为username

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

frontend/src/App.vue

@@ -15,7 +15,7 @@
       <div class="header-right" @click="toggleUserMenu">
         <template v-if="auth.isLoggedIn">
           <span v-if="auth.isBusiness" class="biz-badge" title="商业认证用户">🏢</span>
-          <span class="user-name">{{ auth.user.display_name || auth.user.username }} ▾</span>
+          <span class="user-name">{{ auth.user.username }} ▾</span>
           <span v-if="unreadNotifCount > 0" class="notif-badge">{{ unreadNotifCount }}</span>
         </template>
         <template v-else>

frontend/src/components/LoginModal.vue

@@ -37,14 +37,6 @@
           class="login-input"
           @keydown.enter="submit"
         />
-        <input
-          v-if="mode === 'register'"
-          v-model="displayName"
-          type="text"
-          placeholder="显示名称（可选）"
-          class="login-input"
-          @keydown.enter="submit"
-        />
 
         <div v-if="errorMsg" class="login-error">{{ errorMsg }}</div>
 
@@ -80,7 +72,6 @@ const mode = ref('login')
 const username = ref('')
 const password = ref('')
 const confirmPassword = ref('')
-const displayName = ref('')
 const errorMsg = ref('')
 const loading = ref(false)
 const showFeedback = ref(false)
@@ -109,11 +100,7 @@ async function submit() {
       await auth.login(username.value.trim(), password.value)
       ui.showToast('登录成功')
     } else {
-      await auth.register(
-        username.value.trim(),
-        password.value,
-        displayName.value.trim() || username.value.trim()
-      )
+      await auth.register(username.value.trim(), password.value)
       ui.showToast('注册成功')
     }
     emit('close')

frontend/src/components/UserMenu.vue

@@ -1,7 +1,10 @@
 <template>
   <div class="usermenu-overlay" @mousedown.self="$emit('close')">
     <div class="usermenu-card">
-      <div class="usermenu-name">{{ auth.user.display_name || auth.user.username }}</div>
+      <div class="usermenu-name">
+        {{ auth.user.username }}
+        <button v-if="!auth.user.username_changed" class="rename-btn" @click="changeUsername">✏️ 改名</button>
+      </div>
 
       <div class="usermenu-actions">
         <button class="usermenu-btn" @click="goMyDiary">
@@ -131,6 +134,27 @@ function isSearchMissing(n) {
   return n.title && n.title.includes('用户需求')
 }
 
+async function changeUsername() {
+  const { showPrompt } = await import('../composables/useDialog')
+  const newName = await showPrompt('输入新用户名（只能修改一次）：', auth.user.username)
+  if (!newName || !newName.trim() || newName.trim() === auth.user.username) return
+  try {
+    const res = await api('/api/me/username', {
+      method: 'PUT',
+      body: JSON.stringify({ username: newName.trim() }),
+    })
+    if (res.ok) {
+      await auth.loadMe()
+      ui.showToast('用户名已修改')
+    } else {
+      const err = await res.json().catch(() => ({}))
+      ui.showToast(err.detail || '修改失败')
+    }
+  } catch {
+    ui.showToast('修改失败')
+  }
+}
+
 function isPlanRequest(n) {
   return n.title && (n.title.includes('方案请求') || n.title.includes('方案需求'))
 }
@@ -227,7 +251,9 @@ onMounted(loadNotifications)
   z-index: 4001;
 }
 
-.usermenu-name { font-size: 16px; font-weight: 600; color: #3e3a44; margin-bottom: 14px; }
+.usermenu-name { font-size: 16px; font-weight: 600; color: #3e3a44; margin-bottom: 14px; display: flex; align-items: center; gap: 8px; }
+.rename-btn { background: none; border: none; font-size: 12px; color: #b0aab5; cursor: pointer; padding: 0; }
+.rename-btn:hover { color: #4a9d7e; }
 
 .usermenu-actions { display: flex; flex-direction: column; gap: 4px; }
 .usermenu-btn {

frontend/src/stores/auth.js

@@ -40,9 +40,10 @@ export const useAuthStore = defineStore('auth', () => {
         id: data.id,
         role: data.role,
         username: data.username,
-        display_name: data.display_name,
+        display_name: data.username,
         has_password: data.has_password ?? false,
         business_verified: data.business_verified ?? false,
+        username_changed: data.username_changed ?? false,
       }
     } catch {
       logout()
@@ -56,11 +57,10 @@ export const useAuthStore = defineStore('auth', () => {
     await loadMe()
   }
 
-  async function register(username, password, displayName) {
+  async function register(username, password) {
     const data = await api.post('/api/register', {
       username,
       password,
-      display_name: displayName,
     })
     token.value = data.token
     localStorage.setItem('oil_auth_token', data.token)