feat: 实现站内USDT收银台支付流程

- 后端: payment.go 支付处理(创建订单/回调通知/状态查询/签名算法)
- 后端: 新增公开支付通道API(PublicList,不暴露config)
- 前端: Checkout.vue 收银台页面(倒计时/二维码/支付轮询)
- 前端: Cart.vue 对接动态支付通道,选择bepusdt跳转收银台
- 前端: API添加paymentChannelApi和orderApi支付方法
- 路由: 注册checkout路由和支付相关API

frontend/src/router/index.ts

@@ -18,6 +18,7 @@ const routes = [
       { path: 'cart', name: 'Cart', component: () => import('../views/user/Cart.vue') },
       { path: 'orders', name: 'Orders', component: () => import('../views/user/Orders.vue') },
       { path: 'orders/:id', name: 'OrderDetail', component: () => import('../views/user/OrderDetail.vue') },
+      { path: 'checkout/:id', name: 'Checkout', component: () => import('../views/user/Checkout.vue') },
       { path: 'profile', name: 'Profile', component: () => import('../views/user/Profile.vue') },
       { path: 'addresses', name: 'Addresses', component: () => import('../views/user/Addresses.vue') },
       { path: 'lotteries', name: 'Lotteries', component: () => import('../views/user/Lotteries.vue') },