feat: Stripe 改用 PaymentIntent + Payment Element，iframe 嵌入支付宝弹窗支付

Stripe 集成重构:
- 从 Checkout Session 改为 PaymentIntent + Payment Element 模式
- 前端内联渲染 Stripe 支付表单，支持信用卡、支付宝等多种方式
- Webhook 事件改为 payment_intent.succeeded / payment_intent.payment_failed
- provider/test 同步更新

iframe 嵌入模式 (ui_mode=embedded):
- 支付宝等需跳转的方式改为弹出新窗口处理，避免 X-Frame-Options 冲破 iframe
- 信用卡等无跳转方式仍在 iframe 内联完成
- 弹窗使用 confirmAlipayPayment 直接跳转，无需二次操作
- result 页面检测弹窗模式，支付成功后自动关闭窗口

Bug 修复:
- 修复配置加载前支付方式闪烁（初始值改为空数组 + loading）
- 修复桌面端 PaymentForm 缺少 methodLimits prop
- 修复 stripeError 隐藏表单导致无法重试
- 快捷金额增加 1000/2000 选项，过滤低于 minAmount 的选项

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.en.md

@@ -146,7 +146,7 @@ Any payment provider compatible with the **EasyPay protocol** can be used, such
 | `STRIPE_WEBHOOK_SECRET` | Stripe webhook signing secret (`whsec_...`) |
 
 > Stripe webhook endpoint: `${NEXT_PUBLIC_APP_URL}/api/stripe/webhook`
-> Subscribe to: `checkout.session.completed`, `checkout.session.expired`
+> Subscribe to: `payment_intent.succeeded`, `payment_intent.payment_failed`
 
 ### Business Rules
 
@@ -310,7 +310,7 @@ User submits recharge amount
          ▼
   User completes payment
   ├─ EasyPay → QR code / H5 redirect
-  └─ Stripe  → Checkout Session
+  └─ Stripe  → Payment Element (PaymentIntent)
          │
          ▼
   Payment callback (signature verified) → Order PAID