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/screens/purchase_page.dart

@@ -1,5 +1,7 @@
 // purchase_page.dart — 购买套餐(三档 pro:月/季/年,pay v2 通道)。
 // 档位数据来自 GET /v1/pay/catalog(server 单源);金额仅展示,扣款以 pay 为准。
+// 支付渠道即币种即方式(USDT→加密货币 默认 / 人民币→支付宝),顶部 SegSwitch 选定,
+// 套餐价随渠道币种刷新;点「立即购买」直接下单进支付页(不再底部弹窗选方式)。
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
@@ -9,8 +11,9 @@ import '../pangolin_theme.dart';
 import '../state/payment_provider.dart';
 import '../widgets/pangolin_icons.dart';
 import '../widgets/plan_card.dart';
+import '../widgets/seg_switch.dart';
 
-class PurchaseScreen extends ConsumerWidget {
+class PurchaseScreen extends ConsumerStatefulWidget {
   const PurchaseScreen({super.key, required this.t, this.onBack, this.onOrderCreated, this.embedded = false});
 
   final AppText t;
@@ -19,6 +22,16 @@ class PurchaseScreen extends ConsumerWidget {
   final VoidCallback? onOrderCreated;
   final bool embedded;
 
+  @override
+  ConsumerState<PurchaseScreen> createState() => _PurchaseScreenState();
+}
+
+class _PurchaseScreenState extends ConsumerState<PurchaseScreen> {
+  AppText get t => widget.t;
+
+  // 支付渠道:默认 USDT·加密货币。切换即刷新套餐卡显价币种。
+  PayChannel _channel = PayChannel.usdt;
+
   String _name(String sku) => switch (sku) {
         'pro_month' => t.proMonthly,
         'pro_quarter' => t.proQuarterly,
@@ -26,57 +39,16 @@ class PurchaseScreen extends ConsumerWidget {
         _ => sku,
       };
 
-  String _period(String sku) => switch (sku) {
-        'pro_month' => t.perMonth,
-        'pro_quarter' => t.perQuarter,
-        'pro_year' => t.perYear,
-        _ => '',
-      };
-
-  Future<void> _choose(BuildContext context, WidgetRef ref, PayCatalogItem item) async {
-    final c = context.pangolin;
-    final method = await showModalBottomSheet<String>(
-      context: context,
-      backgroundColor: c.surface,
-      shape: RoundedRectangleBorder(
-        borderRadius: BorderRadius.vertical(top: Radius.circular(PangolinRadius.xl)),
-      ),
-      builder: (sheetCtx) => SafeArea(
-        child: Column(mainAxisSize: MainAxisSize.min, children: [
-          Padding(
-            padding: const EdgeInsets.fromLTRB(20, 18, 20, 8),
-            child: Align(
-              alignment: Alignment.centerLeft,
-              child: Text(t.choosePayMethod,
-                  style: PangolinText.h3.copyWith(color: c.fg1, fontWeight: FontWeight.w700)),
-            ),
-          ),
-          ListTile(
-            leading: Icon(PangolinIcons.creditCard, color: c.fg2),
-            title: Text(t.payMethodAlipay, style: PangolinText.body.copyWith(color: c.fg1)),
-            trailing: Icon(PangolinIcons.chevronRight, size: 18, color: c.fg3),
-            onTap: () => Navigator.of(sheetCtx).pop('alipay'),
-          ),
-          ListTile(
-            leading: Icon(PangolinIcons.globe, color: c.fg2),
-            title: Text(t.payMethodCrypto, style: PangolinText.body.copyWith(color: c.fg1)),
-            trailing: Icon(PangolinIcons.chevronRight, size: 18, color: c.fg3),
-            onTap: () => Navigator.of(sheetCtx).pop('crypto'),
-          ),
-          const SizedBox(height: 12),
-        ]),
-      ),
-    );
-    if (method == null || !context.mounted) return;
-    await ref.read(paymentFlowProvider.notifier).start(item, method);
-    if (!context.mounted) return;
+  Future<void> _buy(PayCatalogItem item) async {
+    await ref.read(paymentFlowProvider.notifier).start(item, _channel.method);
+    if (!mounted) return;
     if (ref.read(paymentFlowProvider).phase == PaymentPhase.awaitingPayment) {
-      onOrderCreated?.call();
+      widget.onOrderCreated?.call();
     }
   }
 
   @override
-  Widget build(BuildContext context, WidgetRef ref) {
+  Widget build(BuildContext context) {
     final c = context.pangolin;
     final catalog = ref.watch(payCatalogProvider);
 
@@ -93,25 +65,42 @@ class PurchaseScreen extends ConsumerWidget {
       data: (items) => ListView(
         padding: const EdgeInsets.fromLTRB(20, 14, 20, 24),
         children: [
+          // 支付渠道 SegSwitch(默认 USDT;人民币走支付宝)→ 套餐价随之刷新
+          Padding(
+            padding: const EdgeInsets.only(left: 2, bottom: 8),
+            child: Text(t.payChannel.toUpperCase(),
+                style: PangolinText.caption.copyWith(
+                    color: c.fg3, fontWeight: FontWeight.w700, letterSpacing: 0.6)),
+          ),
+          SegSwitch(
+            options: [
+              (icon: PangolinIcons.globe, label: t.payChannelUsdt),
+              (icon: PangolinIcons.creditCard, label: t.payChannelCny),
+            ],
+            selectedIndex: _channel == PayChannel.usdt ? 0 : 1,
+            onChanged: (i) => setState(
+                () => _channel = i == 0 ? PayChannel.usdt : PayChannel.cny),
+          ),
+          const SizedBox(height: 18),
           for (final it in items)
             Padding(
               padding: EdgeInsets.only(bottom: 14, top: it.sku == 'pro_year' ? 12 : 0),
               child: PlanCard(
                 name: _name(it.sku),
-                price: it.priceLabel(),
-                period: _period(it.sku),
+                price: it.priceLabel(_channel),
+                period: '${it.perMonthLabel(_channel)}${t.perMonthSuffix}',
                 features: t.featsPro,
                 ctaLabel: t.buyNow,
                 featured: it.sku == 'pro_year',
                 popularLabel: it.sku == 'pro_year' ? t.mostPopular : null,
-                onPressed: () => _choose(context, ref, it),
+                onPressed: () => _buy(it),
               ),
             ),
         ],
       ),
     );
 
-    if (embedded) return body;
+    if (widget.embedded) return body;
     return Scaffold(
       backgroundColor: c.bg,
       body: SafeArea(
@@ -120,7 +109,7 @@ class PurchaseScreen extends ConsumerWidget {
             padding: const EdgeInsets.fromLTRB(8, 6, 16, 10),
             child: Row(children: [
               IconButton(
-                onPressed: onBack ?? () => Navigator.of(context).maybePop(),
+                onPressed: widget.onBack ?? () => Navigator.of(context).maybePop(),
                 icon: Icon(PangolinIcons.arrowLeft, size: 22, color: c.fg1),
               ),
               Text(t.purchaseTitle,