feat(i18n): 切换语言时同步更新页面标题

- resolveDocumentTitle() 新增 titleKey 参数，优先通过 i18n 翻译
- router beforeEach 中将路由 meta.titleKey 传入标题解析函数
- setLocale() 切换语言后同步刷新 document.title

frontend/src/router/index.ts

@@ -41,7 +41,8 @@ const routes: RouteRecordRaw[] = [
     component: () => import('@/views/auth/LoginView.vue'),
     meta: {
       requiresAuth: false,
-      title: 'Login'
+      title: 'Login',
+      titleKey: 'common.login'
     }
   },
   {
@@ -50,7 +51,8 @@ const routes: RouteRecordRaw[] = [
     component: () => import('@/views/auth/RegisterView.vue'),
     meta: {
       requiresAuth: false,
-      title: 'Register'
+      title: 'Register',
+      titleKey: 'auth.createAccount'
     }
   },
   {
@@ -86,7 +88,8 @@ const routes: RouteRecordRaw[] = [
     component: () => import('@/views/auth/ForgotPasswordView.vue'),
     meta: {
       requiresAuth: false,
-      title: 'Forgot Password'
+      title: 'Forgot Password',
+      titleKey: 'auth.forgotPasswordTitle'
     }
   },
   {
@@ -390,7 +393,7 @@ router.beforeEach((to, _from, next) => {
 
   // Set page title
   const appStore = useAppStore()
-  document.title = resolveDocumentTitle(to.meta.title, appStore.siteName)
+  document.title = resolveDocumentTitle(to.meta.title, appStore.siteName, to.meta.titleKey as string)
 
   // Check if route requires authentication
   const requiresAuth = to.meta.requiresAuth !== false // Default to true