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/i18n/locales/zh.ts

@@ -5620,7 +5620,33 @@ export default {
     errors: {
       tooManyPending: '待支付订单过多（最多 {max} 个），请先完成或取消现有订单',
       cancelRateLimited: '取消订单过于频繁，请稍后再试',
+      // Structured error codes (reason strings from backend ApplicationError)
+      PAYMENT_DISABLED: '支付系统已关闭',
+      USER_INACTIVE: '账号已被禁用',
+      BALANCE_PAYMENT_DISABLED: '余额充值功能已关闭',
+      INVALID_AMOUNT: '金额无效',
+      INVALID_INPUT: '参数有误',
+      PLAN_NOT_AVAILABLE: '套餐不存在或已下架',
+      GROUP_NOT_FOUND: '订阅分组不可用',
+      GROUP_TYPE_MISMATCH: '分组类型不是订阅类型',
+      TOO_MANY_PENDING: '待支付订单过多（最多 {max} 个），请先完成或取消现有订单',
+      DAILY_LIMIT_EXCEEDED: '今日充值已达上限，剩余额度 {remaining}',
+      PAYMENT_GATEWAY_ERROR: '支付方式不可用',
+      NO_AVAILABLE_INSTANCE: '暂无可用的支付通道',
+      PAYMENT_PROVIDER_MISCONFIGURED: '支付通道配置错误，请联系管理员',
+      WXPAY_CONFIG_MISSING_KEY: '微信支付配置缺少必填项：{key}',
+      WXPAY_CONFIG_INVALID_KEY_LENGTH: '微信支付 {key} 长度错误，应为 {expected} 字节（实际 {actual}）',
+      WXPAY_CONFIG_INVALID_KEY: '微信支付 {key} 格式错误，请确认复制了完整的 PEM 内容',
       PENDING_ORDERS: '该服务商有未完成的订单，请等待订单完成后再操作',
+      CANCEL_RATE_LIMITED: '取消订单过于频繁，请稍后再试',
+      NOT_FOUND: '订单不存在',
+      FORBIDDEN: '无权限操作此订单',
+      CONFLICT: '订单状态已变更，请刷新',
+      INVALID_ORDER_TYPE: '仅余额订单可申请退款',
+      INVALID_STATUS: '当前订单状态不允许此操作',
+      BALANCE_NOT_ENOUGH: '退款金额超过余额',
+      REFUND_AMOUNT_EXCEEDED: '退款金额超过充值金额',
+      REFUND_FAILED: '退款失败',
     },
     stripePay: '立即支付',
     stripeSuccessProcessing: '支付成功，正在处理订单...',