feat(client): pay v2 支付渠道 switch + 订单列表/详情端到端

购买/支付(Phase B):
- 新 SegSwitch 段控 widget(镜像 .segswitch 原子)
- 购买页重构:顶部支付渠道 switch(默认 USDT→加密货币 / 人民币→支付宝),套餐价随渠道分币种显示,删底部弹窗选方式;点购买直达支付页
- 支付页加订单信息卡(套餐/渠道/金额按币种)
- models:PayChannel + PayCatalogItem.priceUsdtMicro/priceLabel(ch)/perMonthLabel(ch)

订单(P0,全新):
- payment_api.listOrders() + orders_provider(列表 + 详情 family)
- OrdersScreen 列表→详情(状态 pill / 可复制订单号 / 继续支付·重新购买)
- 导航:NavView.orders + desktop_shell 分支 + account 入口

配套:
- l10n +19 getter(支付渠道 4 + 订单 15)×6 语言,l10n 闸绿
- 图标单源迁移 lucide_icons_flutter + pangolin_icons.dart(codegen)
- 联系渠道订正(Telegram/邮件 support@yanmeiai.com/官网) + 前端去广告&时长
- 测试:购买 switch 分币种 + 订单列表/空态/详情 + 假 API listOrders;golden 重录
- flutter analyze 0 error,flutter test 226 全绿

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

client/lib/services/payment_api.dart

@@ -26,6 +26,13 @@ class PaymentApi {
   Future<PayOrderStatus> orderStatus(String orderNo) async =>
       PayOrderStatus.fromJson(await _c.getJson('/v1/pay/orders/$orderNo'));
 
+  /// 订单列表(倒序;纯本地台账,不逐单回查 pay 上游)。
+  Future<List<PayOrderSummary>> listOrders() async {
+    final body = await _c.getJson('/v1/pay/orders');
+    final items = (body['orders'] as List<dynamic>?) ?? const [];
+    return [for (final it in items) PayOrderSummary.fromJson(it as Map<String, dynamic>)];
+  }
+
   Future<PayOrder> retry(String orderNo, {required String method, Map<String, String>? metadata}) async =>
       PayOrder.fromJson(await _c.postJson('/v1/pay/orders/$orderNo/retry', {
         'method': method,