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/screens/account_page.dart

@@ -2,9 +2,11 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../core/responsive/form_factor.dart';
 import '../l10n/app_text.dart';
 import '../pangolin_theme.dart';
 import '../state/app_providers.dart';
+import '../state/navigation_provider.dart';
 import '../widgets/account_screens.dart';
 import '../widgets/app_top_bar.dart';
 import '../widgets/pangolin_icons.dart';
@@ -24,13 +26,20 @@ class AccountPage extends ConsumerWidget {
     final isDark = Theme.of(context).brightness == Brightness.dark;
     final pad = isWide ? 28.0 : 20.0;
 
-    void push(Widget page) =>
-        Navigator.of(context).push(MaterialPageRoute(builder: (_) => page));
+    // desktop 在内容区下钻（切 navView）；mobile/tablet 全屏 push。
+    final isDesktop = context.formFactor == FormFactor.desktop;
+    void open(NavView v, Widget mobilePage) {
+      if (isDesktop) {
+        ref.read(navViewProvider.notifier).state = v;
+      } else {
+        Navigator.of(context).push(MaterialPageRoute(builder: (_) => mobilePage));
+      }
+    }
 
     final body = ListView(
       padding: EdgeInsets.fromLTRB(pad, isWide ? 6 : 0, pad, 24),
       children: [
-        _PlanBanner(t: t, isFree: isFree, onUpgrade: () => push(PlansScreen(t: t))),
+        _PlanBanner(t: t, isFree: isFree, onUpgrade: () => open(NavView.plans, PlansScreen(t: t))),
         const SizedBox(height: 18),
         // 账户信息
         _SectionLabel(text: t.accInfoTitle),
@@ -42,11 +51,11 @@ class AccountPage extends ConsumerWidget {
         const SizedBox(height: 16),
         // 设备 / 兑换 / 联系
         _Card(children: [
-          _NavRow(icon: PangolinIcons.monitorSmartphone, title: t.myDevices, onTap: () => push(DevicesScreen(t: t))),
+          _NavRow(icon: PangolinIcons.monitorSmartphone, title: t.myDevices, onTap: () => open(NavView.devices, DevicesScreen(t: t))),
           Divider(height: 1, color: c.border),
-          _NavRow(icon: PangolinIcons.shoppingBag, title: t.redeemEntry, onTap: () => push(RedeemScreen(t: t))),
+          _NavRow(icon: PangolinIcons.shoppingBag, title: t.redeemEntry, onTap: () => open(NavView.redeem, RedeemScreen(t: t))),
           Divider(height: 1, color: c.border),
-          _NavRow(icon: PangolinIcons.messageCircle, title: t.contactEntry, onTap: () => push(ContactScreen(t: t))),
+          _NavRow(icon: PangolinIcons.messageCircle, title: t.contactEntry, onTap: () => open(NavView.contact, ContactScreen(t: t))),
         ]),
         const SizedBox(height: 16),
         // 语言 / 主题 / 协议