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/widgets/account_screens.dart

@@ -10,15 +10,18 @@ import 'pangolin_button.dart';
 import 'pangolin_icons.dart';
 import 'plan_card.dart';
 
-/// 通用:带返回箭头的子页骨架
+/// 通用:带返回箭头的子页骨架。embedded=true 时只返回内容（desktop 内容区下钻，
+/// 返回/标题由外层 shell 顶栏提供），否则整页 Scaffold（mobile/tablet 全屏 push）。
 class _SubScaffold extends StatelessWidget {
-  const _SubScaffold({required this.title, required this.child, this.onBack});
+  const _SubScaffold({required this.title, required this.child, this.onBack, this.embedded = false});
   final String title;
   final Widget child;
   final VoidCallback? onBack;
+  final bool embedded;
   @override
   Widget build(BuildContext context) {
     final c = context.pangolin;
+    if (embedded) return child;
     return Scaffold(
       backgroundColor: c.bg,
       body: SafeArea(
@@ -42,10 +45,11 @@ class _SubScaffold extends StatelessWidget {
 
 /// ── 套餐选择 ──
 class PlansScreen extends StatelessWidget {
-  const PlansScreen({super.key, required this.t, this.onChoose, this.onBack});
+  const PlansScreen({super.key, required this.t, this.onChoose, this.onBack, this.embedded = false});
   final AppText t;
   final ValueChanged<String>? onChoose;
   final VoidCallback? onBack;
+  final bool embedded;
 
   @override
   Widget build(BuildContext context) {
@@ -57,6 +61,7 @@ class PlansScreen extends StatelessWidget {
     return _SubScaffold(
       title: t.choosePlan,
       onBack: onBack,
+      embedded: embedded,
       child: ListView(
         padding: const EdgeInsets.fromLTRB(20, 14, 20, 24),
         children: [
@@ -84,9 +89,10 @@ class DeviceItem {
 }
 
 class DevicesScreen extends StatefulWidget {
-  const DevicesScreen({super.key, required this.t, this.onBack});
+  const DevicesScreen({super.key, required this.t, this.onBack, this.embedded = false});
   final AppText t;
   final VoidCallback? onBack;
+  final bool embedded;
   @override
   State<DevicesScreen> createState() => _DevicesScreenState();
 }
@@ -104,6 +110,7 @@ class _DevicesScreenState extends State<DevicesScreen> {
     return _SubScaffold(
       title: widget.t.myDevices,
       onBack: widget.onBack,
+      embedded: widget.embedded,
       child: ListView(padding: const EdgeInsets.fromLTRB(20, 4, 20, 24), children: [
         Text(widget.t.devicesSub, style: PangolinText.caption.copyWith(color: c.fg3, fontWeight: FontWeight.w400)),
         const SizedBox(height: 12),
@@ -152,9 +159,10 @@ class _DevicesScreenState extends State<DevicesScreen> {
 
 /// ── 兑换 & 购买 ──
 class RedeemScreen extends StatefulWidget {
-  const RedeemScreen({super.key, required this.t, this.onBack});
+  const RedeemScreen({super.key, required this.t, this.onBack, this.embedded = false});
   final AppText t;
   final VoidCallback? onBack;
+  final bool embedded;
   @override
   State<RedeemScreen> createState() => _RedeemScreenState();
 }
@@ -182,6 +190,7 @@ class _RedeemScreenState extends State<RedeemScreen> {
     return _SubScaffold(
       title: t.redeemTitle,
       onBack: widget.onBack,
+      embedded: widget.embedded,
       child: ListView(padding: const EdgeInsets.fromLTRB(20, 4, 20, 24), children: [
         Container(
           padding: const EdgeInsets.all(18),