feat(client/purchase): 全员优惠月付特殊卡片(¥6/$1,随渠道显价)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/lib/models/payment.dart

@@ -18,6 +18,7 @@ class PayCatalogItem {
     required this.priceMinor,
     required this.currency,
     this.priceUsdtMicro = 0,
+    this.promo = false,
   });
 
   final String sku;
@@ -26,6 +27,7 @@ class PayCatalogItem {
   final int priceMinor;      // CNY 分(展示)
   final String currency;     // CNY
   final int priceUsdtMicro;  // USDT 微元(展示;各币种一口价)
+  final bool promo;          // 全员可见的限时优惠档(如 pro_month_promo)
 
   factory PayCatalogItem.fromJson(Map<String, dynamic> j) => PayCatalogItem(
         sku: j['sku'] as String,
@@ -34,6 +36,7 @@ class PayCatalogItem {
         priceMinor: (j['price_minor'] as num?)?.toInt() ?? 0,
         currency: j['currency'] as String? ?? 'CNY',
         priceUsdtMicro: (j['price_usdt_micro'] as num?)?.toInt() ?? 0,
+        promo: j['promo'] as bool? ?? false,
       );
 
   /// 约当月数(31→1 / 92→3 / 366→12),用于每月折算展示。