feat: 全站多语言支持 (i18n)，lang=en 显示英文，其余默认中文

新增 src/lib/locale.ts 作为统一多语言入口，覆盖前台支付链路、
管理后台、API/服务层错误文案，共 35 个文件。URL 参数 lang 全链路透传，
包括 Stripe return_url、页面跳转、layout html lang 属性等。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/admin/PaymentMethodChart.tsx

@@ -1,6 +1,7 @@
 'use client';
 
 import { getPaymentTypeLabel, getPaymentMeta } from '@/lib/pay-utils';
+import type { Locale } from '@/lib/locale';
 
 interface PaymentMethod {
   paymentType: string;
@@ -12,9 +13,14 @@ interface PaymentMethod {
 interface PaymentMethodChartProps {
   data: PaymentMethod[];
   dark?: boolean;
+  locale?: Locale;
 }
 
-export default function PaymentMethodChart({ data, dark }: PaymentMethodChartProps) {
+export default function PaymentMethodChart({ data, dark, locale = 'zh' }: PaymentMethodChartProps) {
+  const title = locale === 'en' ? 'Payment Method Distribution' : '支付方式分布';
+  const emptyText = locale === 'en' ? 'No data' : '暂无数据';
+  const currency = locale === 'en' ? '$' : '¥';
+
   if (data.length === 0) {
     return (
       <div
@@ -24,9 +30,9 @@ export default function PaymentMethodChart({ data, dark }: PaymentMethodChartPro
         ].join(' ')}
       >
         <h3 className={['mb-4 text-sm font-semibold', dark ? 'text-slate-200' : 'text-slate-800'].join(' ')}>
-          支付方式分布
+          {title}
         </h3>
-        <p className={['text-center text-sm py-8', dark ? 'text-slate-500' : 'text-gray-400'].join(' ')}>暂无数据</p>
+        <p className={['text-center text-sm py-8', dark ? 'text-slate-500' : 'text-gray-400'].join(' ')}>{emptyText}</p>
       </div>
     );
   }
@@ -39,18 +45,18 @@ export default function PaymentMethodChart({ data, dark }: PaymentMethodChartPro
       ].join(' ')}
     >
       <h3 className={['mb-4 text-sm font-semibold', dark ? 'text-slate-200' : 'text-slate-800'].join(' ')}>
-        支付方式分布
+        {title}
       </h3>
       <div className="space-y-4">
         {data.map((method) => {
           const meta = getPaymentMeta(method.paymentType);
-          const label = getPaymentTypeLabel(method.paymentType);
+          const label = getPaymentTypeLabel(method.paymentType, locale);
           return (
             <div key={method.paymentType}>
               <div className="mb-1.5 flex items-center justify-between text-sm">
                 <span className={dark ? 'text-slate-300' : 'text-slate-700'}>{label}</span>
                 <span className={dark ? 'text-slate-400' : 'text-slate-500'}>
-                  ¥{method.amount.toLocaleString()} · {method.percentage}%
+                  {currency}{method.amount.toLocaleString()} · {method.percentage}%
                 </span>
               </div>
               <div