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/state/navigation_provider.dart

@@ -4,7 +4,10 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 /// 一级视图 + 账户子页。
 /// desktop 侧栏暴露 6 个一级项（connect..settings）；mobile/tablet 取前 4 项，
 /// contact/settings 在 mobile 走账户子页。plans/redeem 是 account 的下钻页。
-enum NavView { connect, servers, stats, account, contact, settings, plans, redeem }
+enum NavView { connect, servers, stats, account, contact, settings, plans, redeem, devices }
+
+/// 账户下钻子页（desktop 在内容区切换、顶栏带返回；mobile/tablet 走全屏 push）。
+const Set<NavView> kAccountSubViews = {NavView.plans, NavView.redeem, NavView.devices};
 
 /// 当前视图。
 final navViewProvider = StateProvider<NavView>((ref) => NavView.connect);