修复(#134): 促销前端展示对齐 (expires_at / billing 补 promo_discount / 类型补字段)

Squash merge of fix/134-促销前端展示对齐 (4702dee).

HIF-79/HIF-113 落地后前端有三处对齐缺失：

- promo-info.tsx 仍读 promo.end_time，但后端 SubscribeDiscountPromo
  JSON 字段是 expires_at（×2 处）
- billing.tsx 缺 Promo Discount 行，PreOrderResponse 拿到 promo_discount
  时无处可渲染
- typings.d.ts: SubscribeDiscountPromo 缺 expires_at/rule_type,
  PreOrderResponse 缺 promo_discount

本次仅修字段对齐与展示行，不动业务逻辑、不动 OrderDetail 类型 (后端
当前未透出 promo_discount)。新增 billing.promoDiscount 中英文文案。

packages/ui/src/services/user/typings.d.ts

@@ -548,6 +548,7 @@ declare namespace API {
     price: number;
     amount: number;
     discount: number;
+    promo_discount: number;
     gift_amount: number;
     coupon: string;
     coupon_discount: number;
@@ -1024,8 +1025,9 @@ declare namespace API {
 
   type SubscribeDiscountPromo = {
     rule_name?: string;
+    rule_type?: string;
     promo_price: number;
-    end_time?: number;
+    expires_at?: number;
   };
 
   type SubscribeGroup = {