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。

apps/user/src/sections/subscribe/billing.tsx

@@ -10,6 +10,7 @@ interface SubscribeBillingProps {
       unit_price: number;
       unit_time: string;
       subscribe_discount: number;
+      promo_discount: number;
       show_original_price?: boolean;
     }
   >;
@@ -66,6 +67,14 @@ export function SubscribeBilling({ order }: Readonly<SubscribeBillingProps>) {
             <Display type="currency" value={order?.discount} />
           </span>
         </li>
+        <li>
+          <span className="text-muted-foreground">
+            {t("billing.promoDiscount", "Promo Discount")}
+          </span>
+          <span>
+            <Display type="currency" value={order?.promo_discount} />
+          </span>
+        </li>
         <li>
           <span className="text-muted-foreground">
             {t("billing.couponDiscount", "Coupon Discount")}