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/app/api/admin/orders/[id]/cancel/route.ts

@@ -1,19 +1,26 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { verifyAdminToken, unauthorizedResponse } from '@/lib/admin-auth';
+import { resolveLocale } from '@/lib/locale';
 import { adminCancelOrder } from '@/lib/order/service';
 import { handleApiError } from '@/lib/utils/api';
 
 export async function POST(request: NextRequest, { params }: { params: Promise<{ id: string }> }) {
-  if (!(await verifyAdminToken(request))) return unauthorizedResponse();
+  if (!(await verifyAdminToken(request))) return unauthorizedResponse(request);
+
+  const locale = resolveLocale(request.nextUrl.searchParams.get('lang'));
 
   try {
     const { id } = await params;
-    const outcome = await adminCancelOrder(id);
+    const outcome = await adminCancelOrder(id, locale);
     if (outcome === 'already_paid') {
-      return NextResponse.json({ success: true, status: 'PAID', message: '订单已支付完成' });
+      return NextResponse.json({
+        success: true,
+        status: 'PAID',
+        message: locale === 'en' ? 'Order has already been paid' : '订单已支付完成',
+      });
     }
     return NextResponse.json({ success: true });
   } catch (error) {
-    return handleApiError(error, '取消订单失败');
+    return handleApiError(error, locale === 'en' ? 'Cancel order failed' : '取消订单失败', request);
   }
 }