diff --git a/frontend/src/components/AuthGate.tsx b/frontend/src/components/AuthGate.tsx
index dd3764b..a1e1f3d 100644
--- a/frontend/src/components/AuthGate.tsx
+++ b/frontend/src/components/AuthGate.tsx
@@ -2,10 +2,12 @@ import { createContext, useContext, useEffect, useState } from 'react'
 import type { ReactNode } from 'react'
 import type { User } from '../types'
 
-const SHARED_TIMEOUT_MS = 10 * 60 * 1000
-
-function isSharedDevice() {
-  return document.cookie.split(';').some(c => c.trim() === 'hnf_shared_device=1')
+function getInactivityMs(): number | null {
+  if (window.matchMedia('(display-mode: standalone)').matches) return null
+  const c = document.cookie.split(';').map(s => s.trim()).find(s => s.startsWith('hnf_inactivity_mins='))
+  if (!c) return null
+  const mins = parseInt(c.split('=')[1])
+  return isNaN(mins) || mins <= 0 ? null : mins * 60 * 1000
 }