fix payment qr fallback and admin guidance

docs/PAYMENT.md

@@ -122,7 +122,7 @@ Compatible with any payment service that implements the EasyPay protocol.
 
 ### Alipay (Direct)
 
-Direct integration with Alipay Open Platform. Desktop flows return a QR code for in-page display, while mobile flows return an Alipay WAP/app redirect URL.
+Direct integration with Alipay Open Platform. Mobile flows return an Alipay WAP/app redirect URL. Desktop flows prefer Face-to-Face Precreate QR payloads; if the merchant has not enabled that product, the provider falls back to Computer Website Pay and also returns the cashier URL so the frontend can render a QR code or open the hosted checkout page directly.
 
 | Parameter | Description | Required |
 |-----------|-------------|----------|
@@ -229,7 +229,7 @@ User selects amount and payment method
        ▼
   User completes payment
   ├─ EasyPay     → QR code / H5 redirect
-  ├─ Alipay      → Desktop QR / mobile Alipay redirect
+  ├─ Alipay      → Desktop QR payload (Face-to-Face preferred, Website Pay fallback) / mobile Alipay redirect
   ├─ WeChat Pay  → Desktop Native QR / non-WeChat H5 / in-WeChat JSAPI
   └─ Stripe      → Payment Element (card/Alipay/WeChat/etc.)
        │

docs/PAYMENT_CN.md

@@ -122,7 +122,7 @@ Sub2API 内置支付系统，支持用户自助充值，无需部署独立的支
 
 ### 支付宝官方
 
-直接对接支付宝开放平台。桌面端返回二维码供页面内展示和扫码，移动端返回支付宝手机网站支付跳转链接。
+直接对接支付宝开放平台。移动端走支付宝手机网站支付跳转；桌面端优先使用当面付返回扫码串，若商户未开通当面付则回退到电脑网站支付，并将收银台链接同时返回给前端用于渲染二维码或直接打开支付页。
 
 | 参数 | 说明 | 必填 |
 |------|------|------|
@@ -229,7 +229,7 @@ Sub2API 内置支付系统，支持用户自助充值，无需部署独立的支
        ▼
   用户完成支付
   ├─ EasyPay    → 扫码 / H5 跳转
-  ├─ 支付宝官方  → 桌面二维码 / 移动端支付宝跳转
+  ├─ 支付宝官方  → 桌面扫码单（当面付优先，电脑网站支付回退）/ 移动端支付宝跳转
   ├─ 微信官方    → 桌面 Native 扫码 / 非微信 H5 / 微信内 JSAPI
   └─ Stripe     → Payment Element（银行卡/支付宝/微信等）
        │