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The native Alipay provider previously tried to embed the payment page
URL into a QR code on the client — the URL is not a scannable payload
so the QR never worked. Merchants also hit a H5 detection mismatch
whenever the backend UA sniffer missed iPadOS 13+ or embedded browsers,
and the popup window was too small for Alipay's standard checkout
layout (QR + account-login panel on the right), forcing the user to
scroll horizontally and vertically.

Changes:

Backend
- alipay.go: drop QR-on-URL path. Use redirect-only flow —
  alipay.trade.page.pay for PC (returns a gateway URL the browser
  opens in a new window) and alipay.trade.wap.pay for H5 (returns a
  URL the browser jumps to). Both flows produce pages on
  openapi.alipaydev.com / excashier.alipay.com; the client never
  renders a QR itself.
- payment_handler.go: add optional is_mobile bool to
  CreateOrderRequest so the frontend can declare the device
  explicitly. Server still falls back to UA sniffing when absent.

Frontend
- types/payment.ts, PaymentView.vue: declare is_mobile in
  CreateOrderRequest and pass the computed isMobileDevice() value.
- providerConfig.ts: replace the two fixed POPUP_WINDOW_FEATURES
  constants with getPaymentPopupFeatures(), which prefers 1250×900
  (Alipay's checkout footprint), clamps to window.screen.avail* and
  centers the popup so it never overflows on smaller laptops.
- PaymentQRDialog.vue, PaymentStatusPanel.vue, StripePaymentInline.vue,
  PaymentView.vue: use the new helper at all popup call sites.

fix(payment): fix Alipay/Wxpay direct provider type mapping and enable cross-provider load balancing

Two issues fixed:

1. Alipay.SupportedTypes() returned ["alipay_direct"] and Wxpay returned
   ["wxpay_direct"], but the frontend sends payment_type="alipay"/"wxpay".
   The registry lookup failed with "payment method (alipay) is not
   configured". Fix: return the base types ["alipay"]/["wxpay"].

2. When multiple providers support the same payment type (e.g. EasyPay
   and Alipay direct both handle "alipay"), only the last-registered
   provider's instances were reachable — the registry mapped one type to
   one provider key, and SelectInstance queried by that single key.

   Fix: bypass the registry in invokeProvider and let SelectInstance
   query across all providers when providerKey is empty. The selected
   instance's own ProviderKey (now included in InstanceSelection) is
   used to create the correct provider, enabling true cross-provider
   load balancing.

Closes #1592

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.