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.

backend/internal/handler/payment_handler.go

@@ -206,6 +206,10 @@ type CreateOrderRequest struct {
 	PaymentType string  `json:"payment_type" binding:"required"`
 	OrderType   string  `json:"order_type"`
 	PlanID      int64   `json:"plan_id"`
+	// IsMobile lets the frontend declare its mobile status directly. When
+	// nil we fall back to User-Agent heuristics (which miss iPadOS / some
+	// embedded browsers that strip the "Mobile" keyword).
+	IsMobile *bool `json:"is_mobile,omitempty"`
 }
 
 // CreateOrder creates a new payment order.
@@ -222,12 +226,16 @@ func (h *PaymentHandler) CreateOrder(c *gin.Context) {
 		return
 	}
 
+	mobile := isMobile(c)
+	if req.IsMobile != nil {
+		mobile = *req.IsMobile
+	}
 	result, err := h.paymentService.CreateOrder(c.Request.Context(), service.CreateOrderRequest{
 		UserID:      subject.UserID,
 		Amount:      req.Amount,
 		PaymentType: req.PaymentType,
 		ClientIP:    c.ClientIP(),
-		IsMobile:    isMobile(c),
+		IsMobile:    mobile,
 		SrcHost:     c.Request.Host,
 		SrcURL:      c.Request.Referer(),
 		OrderType:   req.OrderType,