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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.

Backend:
- Use cfg.RechargeFeeRate in order creation instead of hardcoded 0
- Remove dead getFeeRate stub method
- All amounts computed server-side: order_amount, pay_amount, fee_rate

Frontend - PaymentView:
- Read recharge_fee_rate from checkout-info API (not per-method)
- Show fee breakdown only when fee_rate > 0
- Show credited amount only when multiplier ≠ 1

Frontend - Order display (user + admin):
- Fix fee_rate * 100 bug (fee_rate is already a percentage)
- OrderTable: show pay_amount as primary, fee/credited as sub-lines
- AdminOrderDetail: full breakdown (base/fee/paid/credited)
- AdminRefundDialog: label "到账金额" for clarity
- PaymentResultView: show pay_amount with fee info

Types + i18n:
- Add recharge_fee_rate to CheckoutInfoResponse
- Add fee_rate to CreateOrderResult
- Add translations: creditedAmount, fee, baseAmount, includedInPayAmount

- Add recharge_fee_rate system setting (percentage fee on top of recharge amount)
- Full backend chain: config constant, PaymentConfig struct, update validation,
  read/write persistence, DTO, handler GET/PUT responses
- Frontend: settings input with preview, i18n (zh/en), API types
- Fix provider card toggle layout: labels above switches to save width
- Fix Chinese translation: "EasyPay" → "易支付" in provider description

- Add balance_recharge_multiplier system setting (e.g. 1.2 = charge 100 get 120)
- Separate order_amount (credited balance) from pay_amount (actual payment)
- Refund calculates gateway amount proportionally from pay_amount
- Frontend shows both amounts in order details, payment status, refund dialog
- Admin settings UI for configuring recharge multiplier

allow_user_refund:
- Add allow_user_refund field to PaymentProviderInstance ent schema
- Migration 103: ALTER TABLE payment_provider_instances ADD COLUMN
- Cascade logic: disabling refund_enabled auto-disables allow_user_refund
- User refund validation: check provider instance allows user refund
- Admin refund validation: check provider instance allows admin refund
- Subscription refund: deduct days on refund, rollback on failure
- New endpoint: GET /payment/orders/refund-eligible-providers
- Frontend: ToggleSwitch in ProviderCard/Dialog, cascade in SettingsView

Wildcard matching:
- Change findPricingForModel from "longest prefix wins" to "config order
  priority (first match wins)", aligning with channel service behavior

Closes #1607

Backend:
- Split payment_order.go (546→314 lines) into payment_order_lifecycle.go
- Replace magic strings with constants in factory, easypay, webhook handler
- Add rate limit/validity unit constants in payment_order_lifecycle, payment_service
- Fix critical regression: add PaymentEnabled to GetPublicSettings response
- Add missing migration 099_fix_migrated_purchase_menu_label_icon.sql

Frontend:
- Fix StripePopupView.vue: replace `as any` with typed interface, use extractApiErrorMessage
- Fix AdminOrderTable.vue: replace hardcoded column labels with i18n t() calls
- Fix SubscriptionsView.vue: replace hardcoded Today/Tomorrow with i18n
- Extract duplicate statusBadgeClass/canRefund/formatOrderDateTime to orderUtils.ts
- Add missing i18n keys: common.today, common.tomorrow, payment.orders.orderType/actions
- Remove dead PurchaseSubscriptionView.vue (replaced by PaymentView)

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