feat(payment): i18n payment error codes and label localization

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.

frontend/src/components/payment/PaymentQRDialog.vue

@@ -78,7 +78,7 @@ import Icon from '@/components/icons/Icon.vue'
 import { usePaymentStore } from '@/stores/payment'
 import { useAppStore } from '@/stores'
 import { paymentAPI } from '@/api/payment'
-import { extractApiErrorMessage } from '@/utils/apiError'
+import { extractI18nErrorMessage } from '@/utils/apiError'
 import { getPaymentPopupFeatures } from '@/components/payment/providerConfig'
 import type { PaymentOrder } from '@/types/payment'
 import QRCode from 'qrcode'
@@ -222,7 +222,7 @@ async function handleCancel() {
     cleanup()
     emit('close')
   } catch (err: unknown) {
-    appStore.showError(extractApiErrorMessage(err, t('common.error')))
+    appStore.showError(extractI18nErrorMessage(err, t, 'payment.errors', t('common.error')))
   } finally {
     cancelling.value = false
   }

frontend/src/components/payment/PaymentStatusPanel.vue

@@ -124,7 +124,7 @@ import { useI18n } from 'vue-i18n'
 import { usePaymentStore } from '@/stores/payment'
 import { useAppStore } from '@/stores'
 import { paymentAPI } from '@/api/payment'
-import { extractApiErrorMessage } from '@/utils/apiError'
+import { extractI18nErrorMessage } from '@/utils/apiError'
 import { getPaymentPopupFeatures } from '@/components/payment/providerConfig'
 import type { PaymentOrder } from '@/types/payment'
 import Icon from '@/components/icons/Icon.vue'
@@ -242,7 +242,7 @@ async function handleCancel() {
     cleanup()
     outcome.value = 'cancelled'
   } catch (err: unknown) {
-    appStore.showError(extractApiErrorMessage(err, t('common.error')))
+    appStore.showError(extractI18nErrorMessage(err, t, 'payment.errors', t('common.error')))
   } finally {
     cancelling.value = false
   }

frontend/src/components/payment/StripePaymentInline.vue

@@ -67,7 +67,7 @@
 import { ref, onMounted, nextTick } from 'vue'
 import { useI18n } from 'vue-i18n'
 import { useRouter } from 'vue-router'
-import { extractApiErrorMessage } from '@/utils/apiError'
+import { extractI18nErrorMessage } from '@/utils/apiError'
 import { paymentAPI } from '@/api/payment'
 import { useAppStore } from '@/stores'
 import { getPaymentPopupFeatures } from '@/components/payment/providerConfig'
@@ -132,7 +132,7 @@ onMounted(async () => {
       selectedType.value = event.value.type
     })
   } catch (err: unknown) {
-    initError.value = extractApiErrorMessage(err, t('payment.stripeLoadFailed'))
+    initError.value = extractI18nErrorMessage(err, t, 'payment.errors', t('payment.stripeLoadFailed'))
   } finally {
     loading.value = false
   }
@@ -186,7 +186,7 @@ async function handlePay() {
       emit('success')
     }
   } catch (err: unknown) {
-    error.value = extractApiErrorMessage(err, t('payment.result.failed'))
+    error.value = extractI18nErrorMessage(err, t, 'payment.errors', t('payment.result.failed'))
   } finally {
     submitting.value = false
   }
@@ -199,7 +199,7 @@ async function handleCancel() {
     await paymentAPI.cancelOrder(props.orderId)
     emit('back')
   } catch (err: unknown) {
-    appStore.showError(extractApiErrorMessage(err, t('common.error')))
+    appStore.showError(extractI18nErrorMessage(err, t, 'payment.errors', t('common.error')))
   } finally {
     cancelling.value = false
   }

frontend/src/components/payment/providerConfig.ts

@@ -99,9 +99,9 @@ export const PROVIDER_CONFIG_FIELDS: Record<string, ConfigFieldDef[]> = {
     { key: 'mchId', label: '', sensitive: false },
     { key: 'privateKey', label: '', sensitive: true },
     { key: 'apiV3Key', label: '', sensitive: true },
+    { key: 'certSerial', label: '', sensitive: false },
     { key: 'publicKey', label: '', sensitive: true },
-    { key: 'publicKeyId', label: '', sensitive: false, optional: true },
-    { key: 'certSerial', label: '', sensitive: false, optional: true },
+    { key: 'publicKeyId', label: '', sensitive: false },
   ],
   stripe: [
     { key: 'secretKey', label: '', sensitive: true },