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/test/golden/desktop_pages_golden_test.dart

@@ -61,4 +61,8 @@ void main() {
   testWidgets('desktop 账户页', (t) => _shoot(t, NavView.account, 'desktop_account'));
   testWidgets('desktop 联系页', (t) => _shoot(t, NavView.contact, 'desktop_contact'));
   testWidgets('desktop 设置页', (t) => _shoot(t, NavView.settings, 'desktop_settings'));
+  // 账户下钻子页（内容区 + 顶栏返回）
+  testWidgets('desktop 套餐选择', (t) => _shoot(t, NavView.plans, 'desktop_plans'));
+  testWidgets('desktop 兑换购买', (t) => _shoot(t, NavView.redeem, 'desktop_redeem'));
+  testWidgets('desktop 设备管理', (t) => _shoot(t, NavView.devices, 'desktop_devices'));
 }