fix(client): dispose 不再静默取消支付订单,redirect launchUrl 加失败兜底

- PaymentFlowController 新增 stopPolling():只停轮询 Timer,不发远程 cancel、
  不改 state;PaymentScreen.dispose() 改调它,不再复用 cancel()——离开支付页
  (切左导航 tab)不再等同于放弃订单,cancel() 只留给「取消订单」按钮的显式路径。
- redirect 分支 launchUrl 前加 canLaunchUrl 判定 + try-catch,失败时用
  showPangolinToast 给可见提示(新增 l10n openAlipayFailed,zh/en 均补)。
- purchase_page.dart::_choose 下单后严格判 phase == awaitingPayment 才跳转
  支付页,建单失败(failed)不再误跳。
- 回归测试:payment_pages_test.dart 补两条用例区分 dispose(不触发 cancel、
  state 不归 idle)与「取消订单」按钮(远程 cancel + state 归 idle);
  payment_flow_test.dart 补 stopPolling() 单元测试。

client/test/widget/payment_pages_test.dart

@@ -18,9 +18,10 @@ const _items = [
   PayCatalogItem(sku: 'pro_year', plan: 'pro', days: 366, priceMinor: 19999, currency: 'CNY'),
 ];
 
-/// 轮询生命周期测试专用假 API:只记 orderStatus 调用次数,其余按最小实现返回。
+/// 轮询生命周期测试专用假 API:记 orderStatus / cancel 调用次数,其余按最小实现返回。
 class _PollingFakePaymentApi implements PaymentApi {
   int statusCalls = 0;
+  int cancelCalls = 0;
 
   @override
   Future<List<PayCatalogItem>> catalog() async => const [];
@@ -43,7 +44,9 @@ class _PollingFakePaymentApi implements PaymentApi {
       const PayOrder(orderNo: 'pay1', session: PaySession(renderType: 'redirect', payload: {'url': 'https://x'}));
 
   @override
-  Future<void> cancel(String orderNo) async {}
+  Future<void> cancel(String orderNo) async {
+    cancelCalls++;
+  }
 }
 
 void main() {
@@ -134,7 +137,7 @@ void main() {
     expect(find.text(t.payDone), findsOneWidget);
   });
 
-  testWidgets('支付页 dispose 后停止轮询(离开页面不再调 orderStatus)', (tester) async {
+  testWidgets('支付页 dispose 后停止轮询,但不取消订单(离开页面≠放弃订单)', (tester) async {
     final api = _PollingFakePaymentApi();
     final container = ProviderContainer(overrides: [paymentApiProvider.overrideWithValue(api)]);
     addTearDown(container.dispose);
@@ -161,11 +164,43 @@ void main() {
     final callsWhileMounted = api.statusCalls;
     expect(callsWhileMounted, greaterThan(0));
 
-    // 卸载支付页(触发 dispose)→ 停止轮询,之后不应再新增 orderStatus 调用。
+    // 卸载支付页(触发 dispose)→ 只停轮询,之后不应再新增 orderStatus 调用。
     showPage.value = false;
     await tester.pump();
     await tester.pump(const Duration(seconds: 4));
     await tester.pump(const Duration(seconds: 4));
     expect(api.statusCalls, callsWhileMounted, reason: '支付页卸载后轮询应已停止');
+
+    // 关键回归断言:dispose 绝不能是「隐式取消订单」——不发远程 cancel,
+    // controller state 也不应被重置为 idle,订单在后台仍然存活。
+    expect(api.cancelCalls, 0, reason: 'dispose 不应远程取消订单');
+    expect(container.read(paymentFlowProvider).phase, PaymentPhase.awaitingPayment,
+        reason: 'dispose 后订单状态应保持 awaitingPayment,不应被静默重置为 idle');
+    expect(container.read(paymentFlowProvider).order?.orderNo, 'pay1');
+  });
+
+  testWidgets('支付页「取消订单」按钮:确实远程取消 + 状态归 idle(与 dispose 路径区分)', (tester) async {
+    final api = _PollingFakePaymentApi();
+    final container = ProviderContainer(overrides: [paymentApiProvider.overrideWithValue(api)]);
+    addTearDown(container.dispose);
+    await container.read(paymentFlowProvider.notifier).start(_items.first, 'crypto');
+    expect(container.read(paymentFlowProvider).phase, PaymentPhase.awaitingPayment);
+
+    await tester.pumpWidget(UncontrolledProviderScope(
+      container: container,
+      child: MaterialApp(
+        debugShowCheckedModeBanner: false,
+        theme: PangolinTheme.light,
+        home: PaymentScreen(t: t, embedded: true),
+      ),
+    ));
+    await tester.pump();
+
+    await tester.tap(find.text(t.cancelOrder));
+    await tester.pumpAndSettle();
+
+    expect(api.cancelCalls, 1, reason: '显式点「取消订单」应远程取消');
+    expect(container.read(paymentFlowProvider).phase, PaymentPhase.idle,
+        reason: '显式取消后状态应归 idle');
   });
 }