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/components/payment/PaymentStatusPanel.vue

@@ -125,7 +125,7 @@ import { usePaymentStore } from '@/stores/payment'
 import { useAppStore } from '@/stores'
 import { paymentAPI } from '@/api/payment'
 import { extractApiErrorMessage } from '@/utils/apiError'
-import { POPUP_WINDOW_FEATURES } from '@/components/payment/providerConfig'
+import { getPaymentPopupFeatures } from '@/components/payment/providerConfig'
 import type { PaymentOrder } from '@/types/payment'
 import Icon from '@/components/icons/Icon.vue'
 import QRCode from 'qrcode'
@@ -194,7 +194,7 @@ const countdownDisplay = computed(() => {
 
 function reopenPopup() {
   if (props.payUrl) {
-    window.open(props.payUrl, 'paymentPopup', POPUP_WINDOW_FEATURES)
+    window.open(props.payUrl, 'paymentPopup', getPaymentPopupFeatures())
   }
 }