feat(config): 新增用户前端访问开关

新增 `frontend_enable` 配置并接入后台站点设置，
用于控制用户首页、订阅入口及用户侧 API 是否对外开放。

开关关闭时相关用户入口统一返回空 404，
同时保留节点 API、管理后台与外部回调接口可访问。

补充特性测试覆盖默认开启、关闭隐藏与节点接口白名单场景

app/Http/Kernel.php

@@ -72,6 +72,7 @@ class Kernel extends HttpKernel
         'throttle' => \Illuminate\Routing\Middleware\ThrottleRequests::class,
         'verified' => \Illuminate\Auth\Middleware\EnsureEmailIsVerified::class,
         'user' => \App\Http\Middleware\User::class,
+        'user.frontend' => \App\Http\Middleware\EnsureUserFrontendEnabled::class,
         'admin' => \App\Http\Middleware\Admin::class,
         'client' => \App\Http\Middleware\Client::class,
         'staff' => \App\Http\Middleware\Staff::class,