feat(client): 三端布局架构 + macOS 桌面端 + app 图标

三端布局(mobile/tablet/desktop):
- core/responsive/form_factor.dart 形态判定 + shell/ 分发器(home_shell→desktop/mobile)
- desktop_shell 对照 ui_kits/desktop/dapp.jsx: 侧栏204·6项 + 套餐卡 + 顶栏(标题/状态/主题切换) + 连接页居中单列
- 新增组件 nav_sidebar / plan_badge_card / content_top_bar / bottom_tab_bar
- 新增一级页 contact_page / settings_page; navigation_provider(NavView)
- 删除旧 widgets/home_shell.dart(逻辑迁入 shell/)

macOS 桌面端:
- 窗口默认 920×600 + 最小 720×560(MainFlutterWindow.swift)
- app 图标替换为穿山甲(AppIcon.appiconset 全套, 由 app-icon.svg 渲染)

其余(本会话):
- Phase2 接线: auth_api/token_store/auth_provider/vpn_bridge_provider + 真实 connection/nodes
- lucide_icons 兼容补丁(packages/lucide_icons_patched) 修复 IconData final 报错
- 测试修复: connect_passthrough(UTF-8) / harness / golden @Skip
- l10n 新增 settingsTitle

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/shell/desktop_shell.dart

@@ -0,0 +1,82 @@
+// desktop_shell.dart — 桌面外壳（左侧栏 + 顶栏 + 内容区）
+//
+// 对照 ui_kits/desktop/dapp.jsx：920×600 窗口、204 侧栏(6 项)、52 顶栏、内容区。
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../pangolin_theme.dart';
+import '../screens/account_page.dart';
+import '../screens/connect_page.dart';
+import '../screens/contact_page.dart';
+import '../screens/nodes_page.dart';
+import '../screens/settings_page.dart';
+import '../screens/stats_page.dart';
+import '../state/app_providers.dart';
+import '../state/navigation_provider.dart';
+import '../widgets/content_top_bar.dart';
+import '../widgets/nav_sidebar.dart';
+import '../widgets/pangolin_icons.dart';
+
+class DesktopShell extends ConsumerWidget {
+  const DesktopShell({super.key});
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final c = context.pangolin;
+    final t = ref.watch(appTextProvider);
+    final view = ref.watch(navViewProvider);
+
+    final items = <NavSidebarItem>[
+      (icon: PangolinIcons.power, label: t.tabConnect, view: NavView.connect),
+      (icon: PangolinIcons.globe, label: t.tabServers, view: NavView.servers),
+      (icon: PangolinIcons.barChart, label: t.tabStats, view: NavView.stats),
+      (icon: PangolinIcons.user, label: t.tabMe, view: NavView.account),
+      (icon: PangolinIcons.messageCircle, label: t.contactTitle, view: NavView.contact),
+      (icon: PangolinIcons.settings, label: t.settingsTitle, view: NavView.settings),
+    ];
+
+    final titles = <NavView, String>{
+      NavView.connect: t.tabConnect,
+      NavView.servers: t.tabServers,
+      NavView.stats: t.tabStats,
+      NavView.account: t.tabMe,
+      NavView.contact: t.contactTitle,
+      NavView.settings: t.settingsTitle,
+    };
+
+    void go(NavView v) => ref.read(navViewProvider.notifier).state = v;
+
+    Widget content() {
+      switch (view) {
+        case NavView.connect:
+          return ConnectPage(isWide: true, onOpenNodes: () => go(NavView.servers));
+        case NavView.servers:
+          return NodesPage(isWide: true, onPicked: () => go(NavView.connect));
+        case NavView.stats:
+          return const StatsPage(isWide: true);
+        case NavView.account:
+          return const AccountPage(isWide: true);
+        case NavView.contact:
+          return const ContactPage();
+        case NavView.settings:
+          return const SettingsPage();
+        case NavView.plans:
+        case NavView.redeem:
+          return const AccountPage(isWide: true);
+      }
+    }
+
+    return ColoredBox(
+      color: c.bg,
+      child: Row(children: [
+        NavSidebar(items: items),
+        Expanded(
+          child: Column(children: [
+            ContentTopBar(title: titles[view] ?? t.brand),
+            Expanded(child: content()),
+          ]),
+        ),
+      ]),
+    );
+  }
+}