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/lib/l10n/strings_en.dart

@@ -370,6 +370,8 @@ class StringsEn extends AppText {
   @override
   String get openAlipayHint => 'Return after paying; this page refreshes automatically';
   @override
+  String get openAlipayFailed => "Couldn't open Alipay; check it's installed or try again";
+  @override
   String get awaitingPayment => 'Awaiting payment';
   @override
   String get paySucceeded => 'Activated';