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/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)