feat(client): 购买页三档 + 支付页 render_type 多态(crypto/redirect/qr 预留)+ 导航接线

购买页三档 pro(月/季/年,金额来自 GET /v1/pay/catalog)选档 → 选支付方式 → 下单;
支付页按 session.render_type 多态渲染(crypto_address 地址+精确金额+复制、redirect
外链拉起、qr 预留复制兜底),轮询用 Task 6 的 paymentFlowProvider,409
CURRENCY_MISMATCH 走「换方式开新单」。同时收口 paymentFlowProvider 非 autoDispose
带来的轮询生命周期缺口——支付页 dispose 时停轮询(cancel() 延后到微任务执行,避免
在自身 unmount 期间同步改 state 炸 Riverpod 断言)。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

client/lib/shell/desktop_shell.dart

@@ -9,6 +9,8 @@ import '../screens/account_page.dart';
 import '../screens/connect_page.dart';
 import '../screens/contact_page.dart';
 import '../screens/nodes_page.dart';
+import '../screens/payment_page.dart';
+import '../screens/purchase_page.dart';
 import '../screens/settings_page.dart';
 import '../screens/stats_page.dart';
 import '../state/app_providers.dart';
@@ -46,6 +48,8 @@ class DesktopShell extends ConsumerWidget {
       NavView.plans: t.choosePlan,
       NavView.redeem: t.redeemTitle,
       NavView.devices: t.myDevices,
+      NavView.purchase: t.purchaseTitle,
+      NavView.payment: t.paymentTitle,
     };
 
     void go(NavView v) => ref.read(navViewProvider.notifier).state = v;
@@ -66,15 +70,27 @@ class DesktopShell extends ConsumerWidget {
           return const SettingsPage();
         // 账户下钻子页（内容区，顶栏带返回）
         case NavView.plans:
-          return PlansScreen(t: t, embedded: true, onChoose: (_) => go(NavView.redeem));
+          // 只有 pro 走 pay v2 购买通道;team 等仍走既有兑换/联系人工流程。
+          return PlansScreen(t: t, embedded: true, onChoose: (code) => go(code == 'pro' ? NavView.purchase : NavView.redeem));
         case NavView.redeem:
           return RedeemScreen(t: t, embedded: true);
         case NavView.devices:
           return DevicesScreen(t: t, embedded: true);
+        case NavView.purchase:
+          return PurchaseScreen(t: t, embedded: true, onOrderCreated: () => go(NavView.payment));
+        case NavView.payment:
+          return PaymentScreen(t: t, embedded: true, onDone: () => go(NavView.account));
       }
     }
 
-    final isSub = kAccountSubViews.contains(view);
+    // 子页返回目标:payment → purchase(换方式/失败重试仍在购买语境内);
+    // 其余下钻子页（含 purchase 本身）→ account。
+    VoidCallback? backTarget() {
+      if (view == NavView.payment) return () => go(NavView.purchase);
+      if (kAccountSubViews.contains(view)) return () => go(NavView.account);
+      return null;
+    }
+
     return ColoredBox(
       color: c.bg,
       child: Row(children: [
@@ -83,7 +99,7 @@ class DesktopShell extends ConsumerWidget {
           child: Column(children: [
             ContentTopBar(
               title: titles[view] ?? t.brand,
-              onBack: isSub ? () => go(NavView.account) : null,
+              onBack: backTarget(),
             ),
             Expanded(child: content()),
           ]),