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/widgets/bottom_tab_bar.dart

@@ -0,0 +1,52 @@
+// bottom_tab_bar.dart — 移动端底部 Tab（4 项，对照 ui_kits/mobile）
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../pangolin_theme.dart';
+import '../state/app_providers.dart';
+import '../state/navigation_provider.dart';
+import 'pangolin_icons.dart';
+
+class BottomTabBar extends ConsumerWidget {
+  const BottomTabBar({super.key, required this.current, required this.onTap});
+
+  final NavView current;
+  final ValueChanged<NavView> onTap;
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final c = context.pangolin;
+    final t = ref.watch(appTextProvider);
+    final items = <({IconData icon, String label, NavView view})>[
+      (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),
+    ];
+    return Container(
+      decoration: BoxDecoration(
+        color: c.surface,
+        border: Border(top: BorderSide(color: c.border)),
+      ),
+      padding: const EdgeInsets.only(top: 8, bottom: 22),
+      child: Row(children: [
+        for (final item in items)
+          Expanded(
+            child: GestureDetector(
+              behavior: HitTestBehavior.opaque,
+              onTap: () => onTap(item.view),
+              child: Column(mainAxisSize: MainAxisSize.min, children: [
+                Icon(item.icon, size: 22, color: current == item.view ? c.accent : c.fg3),
+                const SizedBox(height: 4),
+                Text(item.label,
+                    style: PangolinText.caption.copyWith(
+                      color: current == item.view ? c.accent : c.fg3,
+                      fontWeight: current == item.view ? FontWeight.w700 : FontWeight.w500,
+                    )),
+              ]),
+            ),
+          ),
+      ]),
+    );
+  }
+}