fix: 增加路由守卫（token 失效自动回登录页）

401 登出后当前页面不跳转，导致主框架停留在异常状态
（如会员状态一直加载）；守卫在认证态变化时导航到 /login 或 /home

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

lib/main.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
+import 'core/auth/auth_provider.dart';
 import 'features/auth/login_page.dart';
 import 'features/cps/cps_product_list_page.dart';
 import 'features/cps/cps_provider.dart';
@@ -28,6 +29,22 @@ class SloganApp extends ConsumerWidget {
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final router = ref.watch(routerProvider);
+    final auth = ref.watch(authProvider);
+    // 路由守卫：token 失效/登出 → 回登录页；token 有效且在登录页 → 进主框架
+    final path =
+        router.routerDelegate.currentConfiguration.uri.path;
+    final authed = auth.value?.authenticated ?? false;
+    if (!auth.isLoading) {
+      if (!authed && path != '/login') {
+        WidgetsBinding.instance.addPostFrameCallback((_) {
+          if (context.mounted) router.go('/login');
+        });
+      } else if (authed && path == '/login') {
+        WidgetsBinding.instance.addPostFrameCallback((_) {
+          if (context.mounted) router.go('/home');
+        });
+      }
+    }
     return MaterialApp.router(
       title: '我的形象穿搭',
       theme: ThemeData(