feat: 套餐有效期支持日/周/月单位，订阅履约改用兑换码流程，UI层次感优化

- Prisma: SubscriptionPlan 新增 validityUnit 字段 (day/week/month)
- 新增 subscription-utils.ts 计算实际天数及格式化显示
- Sub2API client createAndRedeem 支持 subscription 类型 (group_id, validity_days)
- 订阅履约从 assignSubscription 改为 createAndRedeem，在 Sub2API 留痕
- 订单创建动态计算天数（月单位按自然月差值）
- 管理后台表单支持有效期数值+单位下拉
- 前端 ChannelCard 渠道卡片视觉层次优化（模型标签渐变、倍率突出、闪电图标）
- 按量付费 banner 改为渐变背景+底部倍率说明标签
- 帮助/客服信息区块添加到充值、订阅、支付全流程页面
- 移除系统配置独立页面入口，subscriptions API 返回用户信息

src/app/api/admin/subscriptions/route.ts

@@ -1,6 +1,6 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { verifyAdminToken, unauthorizedResponse } from '@/lib/admin-auth';
-import { getUserSubscriptions } from '@/lib/sub2api/client';
+import { getUserSubscriptions, getUser } from '@/lib/sub2api/client';
 
 export async function GET(request: NextRequest) {
   if (!(await verifyAdminToken(request))) return unauthorizedResponse(request);
@@ -18,7 +18,10 @@ export async function GET(request: NextRequest) {
       return NextResponse.json({ error: '无效的 user_id' }, { status: 400 });
     }
 
-    const subscriptions = await getUserSubscriptions(parsedUserId);
+    const [subscriptions, user] = await Promise.all([
+      getUserSubscriptions(parsedUserId),
+      getUser(parsedUserId).catch(() => null),
+    ]);
 
     // 如果提供了 group_id 筛选，过滤结果
     const groupId = searchParams.get('group_id');
@@ -26,7 +29,10 @@ export async function GET(request: NextRequest) {
       ? subscriptions.filter((s) => s.group_id === Number(groupId))
       : subscriptions;
 
-    return NextResponse.json({ subscriptions: filtered });
+    return NextResponse.json({
+      subscriptions: filtered,
+      user: user ? { id: user.id, username: user.username, email: user.email } : null,
+    });
   } catch (error) {
     console.error('Failed to query subscriptions:', error);
     return NextResponse.json({ error: '查询订阅信息失败' }, { status: 500 });