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Pairs with the backend structured payment errors (reason + metadata). The
frontend now maps reason codes to localized messages with metadata as
interpolation variables, and automatically localizes raw config-field names
(e.g. "certSerial" → "证书序列号") using the existing UI-label i18n
namespace.

- frontend/src/utils/apiError.ts
  - extractApiErrorCode now prefers the string `reason` over the numeric HTTP
    `code`; reason is granular enough to drive i18n lookup, HTTP code is not.
  - New extractApiErrorMetadata to pull interpolation params off the error.
  - New extractI18nErrorMessage(err, t, namespace, fallback): looks up
    `<namespace>.<REASON>` in i18n and substitutes metadata. Before
    substitution, `metadata.key` and `metadata.keys` (slash-joined) are
    re-translated through `admin.settings.payment.field_<key>` so users see
    "缺少必填项：证书序列号" instead of "缺少必填项：certSerial".

- frontend/src/i18n/locales/{zh,en}.ts
  - Add payment.errors entries for every structured reason code returned by
    the backend (PAYMENT_DISABLED, INVALID_AMOUNT, TOO_MANY_PENDING,
    DAILY_LIMIT_EXCEEDED, NO_AVAILABLE_INSTANCE, PAYMENT_PROVIDER_MISCONFIGURED,
    WXPAY_CONFIG_MISSING_KEY / INVALID_KEY_LENGTH / INVALID_KEY, NOT_FOUND,
    FORBIDDEN, CONFLICT, INVALID_ORDER_TYPE, INVALID_STATUS,
    BALANCE_NOT_ENOUGH, REFUND_AMOUNT_EXCEEDED, REFUND_FAILED, and more),
    with placeholders for template variables.

- 13 payment-related Vue files
  - Migrate catch-block error reporting from extractApiErrorMessage to
    extractI18nErrorMessage(err, t, 'payment.errors', fallback).
  - Remove the ad-hoc paymentErrorMap computed in SettingsView.vue, which the
    new helper supersedes (it reads i18n directly via t).

- frontend/src/components/payment/providerConfig.ts
  - wxpay: publicKey and publicKeyId are now required (was optional), matching
    the pubkey-only verifier direction; certSerial is already required.

This PR is drop-in safe: reason-preferring extractApiErrorCode is backward
compatible with callers that pass their own i18nMap, and error codes missing
from i18n fall back to the existing message-based path.

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 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

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.