feat(client): 账户子页 desktop 内容区下钻（档1）

- desktop 上套餐选择/兑换购买/设备管理改为内容区下钻（侧栏保留、shell 顶栏带
  返回箭头），不再全屏 push 盖住侧栏；mobile/tablet 仍全屏 push
- _SubScaffold 加 embedded 参数(true 只返回内容)，PlansScreen/DevicesScreen/
  RedeemScreen 透传；NavView 加 devices + kAccountSubViews 集合
- desktop_shell 渲染子页(embedded) + 顶栏 onBack→account；account_page 按
  formFactor 选下钻(切 navView)或 push
- desktop 整页 golden 补 套餐/兑换/设备 三子页(共 8 页)

测试: flutter test 84 通过(+3 子页 golden), analyze 0 error

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/shell/desktop_shell.dart

@@ -13,6 +13,7 @@ import '../screens/settings_page.dart';
 import '../screens/stats_page.dart';
 import '../state/app_providers.dart';
 import '../state/navigation_provider.dart';
+import '../widgets/account_screens.dart';
 import '../widgets/content_top_bar.dart';
 import '../widgets/nav_sidebar.dart';
 import '../widgets/pangolin_icons.dart';
@@ -42,6 +43,9 @@ class DesktopShell extends ConsumerWidget {
       NavView.account: t.tabMe,
       NavView.contact: t.contactTitle,
       NavView.settings: t.settingsTitle,
+      NavView.plans: t.choosePlan,
+      NavView.redeem: t.redeemTitle,
+      NavView.devices: t.myDevices,
     };
 
     void go(NavView v) => ref.read(navViewProvider.notifier).state = v;
@@ -60,19 +64,27 @@ class DesktopShell extends ConsumerWidget {
           return const ContactPage();
         case NavView.settings:
           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));
         case NavView.redeem:
-          return const AccountPage(isWide: true);
+          return RedeemScreen(t: t, embedded: true);
+        case NavView.devices:
+          return DevicesScreen(t: t, embedded: true);
       }
     }
 
+    final isSub = kAccountSubViews.contains(view);
     return ColoredBox(
       color: c.bg,
       child: Row(children: [
         NavSidebar(items: items),
         Expanded(
           child: Column(children: [
-            ContentTopBar(title: titles[view] ?? t.brand),
+            ContentTopBar(
+              title: titles[view] ?? t.brand,
+              onBack: isSub ? () => go(NavView.account) : null,
+            ),
             Expanded(child: content()),
           ]),
         ),