fix(client/orders): 继续支付改为 retry 恢复原单会话(原误进购买页致重复下单);409 已付自纠刷新

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P9G7E3wmAYL9KeYCVZVsqu

client/lib/state/payment_provider.dart

@@ -112,6 +112,41 @@ class PaymentFlowController extends StateNotifier<PaymentFlowState> {
     }
   }
 
+  /// 恢复 `created` 单的支付会话(订单页「继续支付」)——**不是**新下单。
+  /// 走 retry 拿回(或续出)同一 orderNo 的支付会话;409 ORDER_NOT_PENDING 说明
+  /// 订单在这期间已变化(常见:webhook 已到账但列表还没刷新)→ 落 failed 自纠,
+  /// 由调用方(订单页)据此刷新列表,用户会看到订单其实已开通,不再重复下单。
+  Future<void> resume(String orderNo, String method) async {
+    _stopPolling();
+    state = PaymentFlowState(phase: PaymentPhase.creating, method: method);
+    try {
+      final next = await _ref!
+          .read(paymentApiProvider)
+          .retry(orderNo, method: method, metadata: _metadata(method));
+      if (!mounted) return;
+      state = state.copyWith(phase: PaymentPhase.awaitingPayment, order: next);
+      _startPolling();
+    } on AuthApiException catch (e) {
+      if (!mounted) return;
+      if (e.code == 'ORDER_NOT_PENDING') {
+        state = state.copyWith(
+          phase: PaymentPhase.failed,
+          errorZh: '订单状态已变化，请刷新查看最新状态',
+          errorEn: 'Order status has changed, please refresh to see the latest status',
+        );
+        return;
+      }
+      state = state.copyWith(phase: PaymentPhase.failed, errorZh: e.messageZh, errorEn: e.messageEn);
+    } catch (_) {
+      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',
+      );
+    }
+  }
+
   /// 换支付方式:先 retry;pay 回 409 CURRENCY_MISMATCH(跨结算币种)时,
   /// 按契约「换币种必须新单」→ cancel 旧单 + 新 method 重新下单。
   Future<void> switchMethod(String method) async {