fix(payment): alipay redirect-only flow, H5 detection and popup sizing

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.

frontend/src/types/payment.ts

@@ -154,6 +154,7 @@ export interface CreateOrderRequest {
   payment_type: string
   order_type: string
   plan_id?: number
+  is_mobile?: boolean
 }
 
 export interface CreateOrderResult {