feat(client): tablet 外壳 + 视觉 diff 工作流文档

- tablet_shell: 侧栏 232·4 项触控(minHeight 48, 字号 15) + 顶栏无主题切换 + 连接页双栏
  对照 ui_kits/tablet/tabapp.jsx；home_shell 分发 tablet→TabletShell
- nav_sidebar 加 dense 参数(desktop 紧凑 / tablet 触控)
- content_top_bar 加 showThemeToggle + titleSize 参数
- tool/visual-diff.md: 截图 diff 工作流(design-distill 工具)+ 实现侧三路径
  现状(golden 被 google_fonts 阻塞 / screencapture 受 macOS chrome 限制)+ 推荐

注: tablet 形态在 macOS 桌面平台不触发(只 desktop/mobile)，运行验证需 iPad。

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

client/lib/shell/home_shell.dart

@@ -10,6 +10,7 @@ import '../core/responsive/form_factor.dart';
 import '../pangolin_theme.dart';
 import 'desktop_shell.dart';
 import 'mobile_shell.dart';
+import 'tablet_shell.dart';
 
 class HomeShell extends ConsumerWidget {
   const HomeShell({super.key});
@@ -19,7 +20,7 @@ class HomeShell extends ConsumerWidget {
     final c = context.pangolin;
     final Widget body = switch (context.formFactor) {
       FormFactor.desktop => const DesktopShell(),
-      FormFactor.tablet => const MobileShell(), // TODO: TabletShell
+      FormFactor.tablet => const TabletShell(),
       FormFactor.mobile => const MobileShell(),
     };
     return Scaffold(backgroundColor: c.bg, body: body);