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/OrderSummaryCards.tsx

@@ -1,3 +1,5 @@
+import type { Locale } from '@/lib/locale';
+
 interface Summary {
   total: number;
   pending: number;
@@ -7,32 +9,47 @@ interface Summary {
 
 interface OrderSummaryCardsProps {
   isDark: boolean;
+  locale: Locale;
   summary: Summary;
 }
 
-export default function OrderSummaryCards({ isDark, summary }: OrderSummaryCardsProps) {
+export default function OrderSummaryCards({ isDark, locale, summary }: OrderSummaryCardsProps) {
   const cardClass = [
     'rounded-xl border p-3',
     isDark ? 'border-slate-700 bg-slate-800/70' : 'border-slate-200 bg-slate-50',
   ].join(' ');
   const labelClass = ['text-xs', isDark ? 'text-slate-400' : 'text-slate-500'].join(' ');
+  const labels =
+    locale === 'en'
+      ? {
+          total: 'Total Orders',
+          pending: 'Pending',
+          completed: 'Completed',
+          failed: 'Closed/Failed',
+        }
+      : {
+          total: '总订单',
+          pending: '待支付',
+          completed: '已完成',
+          failed: '异常/关闭',
+        };
 
   return (
     <div className="mb-4 grid grid-cols-2 gap-3 sm:grid-cols-4">
       <div className={cardClass}>
-        <div className={labelClass}>总订单</div>
+        <div className={labelClass}>{labels.total}</div>
         <div className="mt-1 text-xl font-semibold">{summary.total}</div>
       </div>
       <div className={cardClass}>
-        <div className={labelClass}>待支付</div>
+        <div className={labelClass}>{labels.pending}</div>
         <div className="mt-1 text-xl font-semibold">{summary.pending}</div>
       </div>
       <div className={cardClass}>
-        <div className={labelClass}>已完成</div>
+        <div className={labelClass}>{labels.completed}</div>
         <div className="mt-1 text-xl font-semibold">{summary.completed}</div>
       </div>
       <div className={cardClass}>
-        <div className={labelClass}>异常/关闭</div>
+        <div className={labelClass}>{labels.failed}</div>
         <div className="mt-1 text-xl font-semibold">{summary.failed}</div>
       </div>
     </div>