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/core/responsive/form_factor.dart

@@ -0,0 +1,34 @@
+// form_factor.dart — 三端形态判定（mobile / tablet / desktop）
+//
+// 决定外壳与页面布局走哪一套。用 defaultTargetPlatform（web 安全，不依赖 dart:io）
+// + 窗口宽度联合判定：桌面平台优先 desktop，窗口拉极窄时降级 mobile 避免溢出。
+import 'package:flutter/foundation.dart';
+import 'package:flutter/widgets.dart';
+
+enum FormFactor { mobile, tablet, desktop }
+
+bool get _isDesktopPlatform {
+  if (kIsWeb) return false;
+  switch (defaultTargetPlatform) {
+    case TargetPlatform.macOS:
+    case TargetPlatform.windows:
+    case TargetPlatform.linux:
+      return true;
+    default:
+      return false;
+  }
+}
+
+extension FormFactorContext on BuildContext {
+  /// 当前形态。桌面平台(macOS/Win/Linux)且宽≥680 → desktop；触屏/Web 宽≥600 → tablet；否则 mobile。
+  FormFactor get formFactor {
+    final w = MediaQuery.sizeOf(this).width;
+    if (_isDesktopPlatform) return w >= 680 ? FormFactor.desktop : FormFactor.mobile;
+    if (w >= 600) return FormFactor.tablet;
+    return FormFactor.mobile;
+  }
+
+  bool get isDesktop => formFactor == FormFactor.desktop;
+  bool get isTablet => formFactor == FormFactor.tablet;
+  bool get isMobile => formFactor == FormFactor.mobile;
+}