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/widgets/content_top_bar.dart

@@ -12,10 +12,12 @@ import '../state/nodes_provider.dart';
 import 'pangolin_icons.dart';
 
 class ContentTopBar extends ConsumerWidget {
-  const ContentTopBar({super.key, required this.title, this.onBack});
+  const ContentTopBar({super.key, required this.title, this.onBack, this.showThemeToggle = true, this.titleSize = 17});
 
   final String title;
   final VoidCallback? onBack;
+  final bool showThemeToggle;
+  final double titleSize;
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
@@ -40,7 +42,7 @@ class ContentTopBar extends ConsumerWidget {
           const SizedBox(width: 4),
         ],
         Text(title,
-            style: PangolinText.h3.copyWith(color: c.fg1, fontWeight: FontWeight.w700, fontSize: 17)),
+            style: PangolinText.h3.copyWith(color: c.fg1, fontWeight: FontWeight.w700, fontSize: titleSize)),
         const Spacer(),
         Text(
           conn.phase == VpnPhase.on ? '● ${node.code}' : '○',
@@ -50,16 +52,18 @@ class ContentTopBar extends ConsumerWidget {
             color: conn.phase == VpnPhase.on ? c.success : c.fg3,
           ),
         ),
-        const SizedBox(width: 12),
-        InkWell(
-          onTap: () => ref.read(themeModeProvider.notifier).state =
-              isDark ? ThemeMode.light : ThemeMode.dark,
-          borderRadius: BorderRadius.circular(PangolinRadius.sm),
-          child: Padding(
-            padding: const EdgeInsets.all(6),
-            child: Icon(isDark ? PangolinIcons.sun : PangolinIcons.moon, size: 18, color: c.fg2),
+        if (showThemeToggle) ...[
+          const SizedBox(width: 12),
+          InkWell(
+            onTap: () => ref.read(themeModeProvider.notifier).state =
+                isDark ? ThemeMode.light : ThemeMode.dark,
+            borderRadius: BorderRadius.circular(PangolinRadius.sm),
+            child: Padding(
+              padding: const EdgeInsets.all(6),
+              child: Icon(isDark ? PangolinIcons.sun : PangolinIcons.moon, size: 18, color: c.fg2),
+            ),
           ),
-        ),
+        ],
       ]),
     );
   }