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/package.json

@@ -14,10 +14,12 @@
   },
   "dependencies": {
     "@element-plus/icons-vue": "^2.3.2",
+    "@types/qrcode": "^1.5.6",
     "@vitejs/plugin-vue": "^6.0.5",
     "axios": "^1.15.0",
     "element-plus": "^2.13.6",
     "pinia": "^3.0.4",
+    "qrcode": "^1.5.4",
     "sass": "^1.99.0",
     "vue-i18n": "^9.14.5",
     "vue-router": "^4.6.4"