diff --git a/src/web/App.mobile-layout.test.tsx b/src/web/App.mobile-layout.test.tsx
new file mode 100644
index 0000000..a80cada
--- /dev/null
+++ b/src/web/App.mobile-layout.test.tsx
@@ -0,0 +1,12 @@
+import { describe, expect, it } from 'vitest';
+import { readFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+
+describe('App mobile layout', () => {
+  it('sets data-layout attribute when switching layouts', () => {
+    const source = readFileSync(resolve(process.cwd(), 'src/web/App.tsx'), 'utf8');
+
+    expect(source).toContain('useIsMobile(768)');
+    expect(source).toContain("document.documentElement.setAttribute('data-layout'");
+  });
+});
diff --git a/src/web/App.tsx b/src/web/App.tsx
index d7b723f..ebdbec2 100644
--- a/src/web/App.tsx
+++ b/src/web/App.tsx
@@ -16,6 +16,7 @@ import { I18nProvider, useI18n } from './i18n.js';
 import { resolveLoginErrorMessage } from './loginError.js';
 import { SITE_DOCS_URL } from './docsLink.js';
 import { useAnimatedVisibility } from './components/useAnimatedVisibility.js';
+import { useIsMobile } from './components/useIsMobile.js';
 const Dashboard = lazy(() => import('./pages/Dashboard.js'));
 const Sites = lazy(() => import('./pages/Sites.js'));
 const Accounts = lazy(() => import('./pages/Accounts.js'));
@@ -383,6 +384,7 @@ function AppShell() {
   const notifBtnRef = useRef<HTMLButtonElement>(null);
   const latestTaskEventIdRef = useRef(0);
   const toast = useToast();
+  const isMobile = useIsMobile(768);
   const resolvedTheme: 'light' | 'dark' = themeMode === 'system'
     ? (systemPrefersDark ? 'dark' : 'light')
     : themeMode;
@@ -415,6 +417,10 @@ function AppShell() {
     }
   }, [resolvedTheme, themeMode]);
 
+  useEffect(() => {
+    document.documentElement.setAttribute('data-layout', isMobile ? 'mobile' : 'desktop');
+  }, [isMobile]);
+
   useEffect(() => {
     const handler = (e: KeyboardEvent) => {
       if ((e.ctrlKey || e.metaKey) && e.key === 'k') {
diff --git a/src/web/components/useIsMobile.ts b/src/web/components/useIsMobile.ts
new file mode 100644
index 0000000..38161c3
--- /dev/null
+++ b/src/web/components/useIsMobile.ts
@@ -0,0 +1,16 @@
+import { useEffect, useState } from 'react';
+
+export function useIsMobile(breakpoint = 768) {
+  const [isMobile, setIsMobile] = useState(() => (
+    typeof window !== 'undefined' ? window.innerWidth < breakpoint : false
+  ));
+
+  useEffect(() => {
+    const update = () => setIsMobile(window.innerWidth < breakpoint);
+    update();
+    window.addEventListener('resize', update);
+    return () => window.removeEventListener('resize', update);
+  }, [breakpoint]);
+
+  return isMobile;
+}