fix(client/pay): CNY 渠道 method 对齐生产哪吒(nezha)+ 下单失败不再静默

根因(购买按钮全端「点了没反应」):
- 生产 pay-v2 的 CNY 渠道是哪吒(nezha)聚合(底层支付宝/微信),客户端却给
  CNY 发 method=alipay → pay 不认 → POST /api/v2/orders 400。USDT/crypto 一直正常。
- 且 _buy 只在 awaitingPayment 时导航,下单失败(phase=failed)既不导航也不弹错,
  非 Auth 异常还直接从 start() 抛出未捕获 → 用户看着「没反应」。

改:
- PayChannelX.method:cny 'alipay' → 'nezha'(usdt 仍 crypto)
- payment_provider._metadata 法币条件同步 nezha;start() 补 catch(_) 兜非 Auth 异常
  落 failed(状态机不再卡 creating)
- purchase_page._buy:phase==failed 时 showPangolinToast 显式提示
- payment_page 换支付方式的另一渠道 alipay → nezha
- 测试钉:PayChannel.cny.method == 'nezha'(发错 method 会被 pay 400)

nezha 返回 render_type=redirect(payload.url),客户端现成 redirect 分支直接接;
orders_page 早有 'nezha'→payMethodNezha 标签。服务端无需改(method 透传、
SettlementCurrency 只区分 crypto vs 法币)。

验证:flutter analyze 干净;payment 相关 18 测试全过。

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

client/lib/models/payment.dart

@@ -7,7 +7,8 @@ enum PayChannel { usdt, cny }
 
 extension PayChannelX on PayChannel {
   String get currency => this == PayChannel.usdt ? 'USDT' : 'CNY';
-  String get method => this == PayChannel.usdt ? 'crypto' : 'alipay';
+  // CNY 渠道生产走哪吒(nezha)聚合(底层支付宝/微信),非直连 alipay 商户。
+  String get method => this == PayChannel.usdt ? 'crypto' : 'nezha';
 }
 
 class PayCatalogItem {

client/lib/screens/payment_page.dart

@@ -244,7 +244,7 @@ class _PaymentScreenState extends ConsumerState<PaymentScreen> {
   }
 
   Future<void> _pickAndSwitch(BuildContext context, WidgetRef ref, PaymentFlowState s) async {
-    final other = s.method == 'crypto' ? 'alipay' : 'crypto';
+    final other = s.method == 'crypto' ? 'nezha' : 'crypto';
     await ref.read(paymentFlowProvider.notifier).switchMethod(other);
   }
 

client/lib/screens/purchase_page.dart

@@ -10,6 +10,7 @@ import '../models/payment.dart';
 import '../pangolin_theme.dart';
 import '../state/payment_provider.dart';
 import '../widgets/pangolin_icons.dart';
+import '../widgets/pangolin_toast.dart';
 import '../widgets/plan_card.dart';
 import '../widgets/seg_switch.dart';
 
@@ -50,8 +51,17 @@ class _PurchaseScreenState extends ConsumerState<PurchaseScreen> {
   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) {
+    final st = ref.read(paymentFlowProvider);
+    if (st.phase == PaymentPhase.awaitingPayment) {
       widget.onOrderCreated?.call();
+    } else if (st.phase == PaymentPhase.failed) {
+      // 下单失败给可见 toast,别让用户以为「点了没反应」。
+      showPangolinToast(
+        context,
+        t.lang == AppLang.zh
+            ? (st.errorZh ?? '下单失败，请稍后重试')
+            : (st.errorEn ?? 'Failed to create order, please retry'),
+      );
     }
   }
 

client/lib/state/payment_provider.dart

@@ -36,7 +36,7 @@ class PaymentFlowState {
 
   final PaymentPhase phase;
   final PayCatalogItem? item;
-  final String method; // alipay | crypto
+  final String method; // nezha | crypto
   final PayOrder? order;
   final PayOrderStatus? status;
   final String? errorZh;
@@ -82,8 +82,10 @@ class PaymentFlowController extends StateNotifier<PaymentFlowState> {
     return Platform.isAndroid || Platform.isIOS;
   }
 
+  // 法币渠道(nezha)带 is_mobile 提示;crypto 不需要。经 pangolin 代理白名单
+  // (is_mobile/render)透传到 pay。
   Map<String, String> _metadata(String method) =>
-      method == 'alipay' ? {'is_mobile': _isMobile ? '1' : '0'} : const {};
+      method == 'nezha' ? {'is_mobile': _isMobile ? '1' : '0'} : const {};
 
   Future<void> start(PayCatalogItem item, String method) async {
     _stopPolling();
@@ -98,6 +100,15 @@ class PaymentFlowController extends StateNotifier<PaymentFlowState> {
     } on AuthApiException catch (e) {
       if (!mounted) return;
       state = state.copyWith(phase: PaymentPhase.failed, errorZh: e.messageZh, errorEn: e.messageEn);
+    } catch (_) {
+      // 网络/解析等非 Auth 异常也要落 failed —— 否则状态机卡在 creating,
+      // 购买页看着「点了没反应」。
+      if (!mounted) return;
+      state = state.copyWith(
+        phase: PaymentPhase.failed,
+        errorZh: '下单失败，请检查网络后重试',
+        errorEn: 'Failed to create order, please check your network and retry',
+      );
     }
   }
 

client/test/unit/payment_api_test.dart

@@ -29,6 +29,14 @@ void main() {
     expect(items.first.priceLabel(PayChannel.cny), '¥29.99');
   });
 
+  test('PayChannel 渠道→method 映射:usdt→crypto / cny→nezha(生产渠道码)', () {
+    // 回归钉:CNY 生产走哪吒(nezha)聚合,不是直连 alipay——发错 method 会被 pay 回 400。
+    expect(PayChannel.usdt.method, 'crypto');
+    expect(PayChannel.cny.method, 'nezha');
+    expect(PayChannel.usdt.currency, 'USDT');
+    expect(PayChannel.cny.currency, 'CNY');
+  });
+
   test('createOrder 只传 sku+method+metadata,解析 session', () async {
     final api = PaymentApi(_client(MockClient((req) async {
       expect(req.url.path, '/v1/pay/orders');