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/refund/route.ts

@@ -3,6 +3,7 @@ import { z } from 'zod';
 import { verifyAdminToken, unauthorizedResponse } from '@/lib/admin-auth';
 import { processRefund } from '@/lib/order/service';
 import { handleApiError } from '@/lib/utils/api';
+import { resolveLocale } from '@/lib/locale';
 
 const refundSchema = z.object({
   order_id: z.string().min(1),
@@ -11,24 +12,30 @@ const refundSchema = z.object({
 });
 
 export async function POST(request: NextRequest) {
-  if (!(await verifyAdminToken(request))) return unauthorizedResponse();
+  if (!(await verifyAdminToken(request))) return unauthorizedResponse(request);
+
+  const locale = resolveLocale(request.nextUrl.searchParams.get('lang'));
 
   try {
     const body = await request.json();
     const parsed = refundSchema.safeParse(body);
 
     if (!parsed.success) {
-      return NextResponse.json({ error: '参数错误', details: parsed.error.flatten().fieldErrors }, { status: 400 });
+      return NextResponse.json(
+        { error: locale === 'en' ? 'Invalid parameters' : '参数错误', details: parsed.error.flatten().fieldErrors },
+        { status: 400 },
+      );
     }
 
     const result = await processRefund({
       orderId: parsed.data.order_id,
       reason: parsed.data.reason,
       force: parsed.data.force,
+      locale,
     });
 
     return NextResponse.json(result);
   } catch (error) {
-    return handleApiError(error, '退款失败');
+    return handleApiError(error, locale === 'en' ? 'Refund failed' : '退款失败', request);
   }
 }