feat(routing): 设置入口改分流规则下钻行 + 双形态导航

- settings_page.dart: smartRoute 布尔开关行 → 分流规则下钻行(标题 + 当前模式 pill
  + chevron),点击 desktop/tablet 切 navViewProvider 内容区下钻,mobile 全屏 push
  RoutingScreen(与 account_page.dart 的 open() 双形态导航同一范式)。
- navigation_provider.dart: 新增 NavView.routing + kSettingsSubViews 集合(登记
  settings 的下钻子页,供 shell backTarget 判断返回 settings 而非 account)。
- desktop_shell.dart/tablet_shell.dart: 内容区 switch 挂 RoutingScreen(embedded:
  true),backTarget 优先判 kSettingsSubViews → 回 settings。
- settings_ia_test.dart: 追加下钻行断言(标题+模式pill+点击导航到 RoutingScreen)。
- 重录 desktop_settings golden(唯一因本刀布局变化受影响的 golden)。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/shell/desktop_shell.dart

@@ -22,6 +22,7 @@ import '../widgets/account_screens.dart';
 import '../widgets/content_top_bar.dart';
 import '../widgets/nav_sidebar.dart';
 import '../widgets/pangolin_icons.dart';
+import '../widgets/routing_screen.dart';
 
 class DesktopShell extends ConsumerWidget {
   const DesktopShell({super.key});
@@ -56,6 +57,7 @@ class DesktopShell extends ConsumerWidget {
       NavView.orders: t.ordersTitle,
       NavView.invite: t.inviteTitle,
       NavView.notifications: t.notifTitle,
+      NavView.routing: t.routingRulesTitle,
     };
 
     void go(NavView v) => ref.read(navViewProvider.notifier).state = v;
@@ -89,14 +91,18 @@ class DesktopShell extends ConsumerWidget {
           return InviteScreen(t: t, embedded: true);
         case NavView.notifications:
           return NotificationsScreen(t: t, embedded: true);
+        // 设置下钻子页（内容区，顶栏带返回到 settings）
+        case NavView.routing:
+          return const RoutingScreen(embedded: true);
       }
     }
 
     // 子页返回目标:payment → purchase(换方式/失败重试仍在购买语境内);
-    // 其余下钻子页（含 purchase 本身）→ account。
+    // 设置下钻子页 → settings;其余下钻子页（含 purchase 本身）→ account。
     VoidCallback? backTarget() {
       if (view == NavView.payment) return () => go(NavView.purchase);
       if (view == NavView.notifications) return () => go(NavView.account);
+      if (kSettingsSubViews.contains(view)) return () => go(NavView.settings);
       if (kAccountSubViews.contains(view)) return () => go(NavView.account);
       return null;
     }

client/lib/shell/tablet_shell.dart

@@ -18,6 +18,7 @@ import '../state/navigation_provider.dart';
 import '../widgets/content_top_bar.dart';
 import '../widgets/nav_sidebar.dart';
 import '../widgets/pangolin_icons.dart';
+import '../widgets/routing_screen.dart';
 
 class TabletShell extends ConsumerWidget {
   const TabletShell({super.key});
@@ -46,6 +47,7 @@ class TabletShell extends ConsumerWidget {
       NavView.contact: t.contactTitle,
       NavView.invite: t.inviteTitle,
       NavView.notifications: t.notifTitle,
+      NavView.routing: t.routingRulesTitle,
     };
 
     void go(NavView v) => ref.read(navViewProvider.notifier).state = v;
@@ -64,14 +66,17 @@ class TabletShell extends ConsumerWidget {
           return const ContactPage();
         case NavView.notifications:
           return NotificationsScreen(t: t, embedded: true);
+        case NavView.routing:
+          return const RoutingScreen(embedded: true);
         default: // account / plans / redeem / devices → 账户（其余下钻在账户页内）
           return const AccountPage(isWide: true);
       }
     }
 
-    // 通知子页返回目标:回账户(镜像桌面 desktop_shell.dart::backTarget)。
+    // 通知子页返回目标:回账户;设置下钻子页 → settings(镜像桌面 desktop_shell.dart::backTarget)。
     VoidCallback? backTarget() {
       if (view == NavView.notifications) return () => go(NavView.account);
+      if (kSettingsSubViews.contains(view)) return () => go(NavView.settings);
       return null;
     }