Add shared device inactivity timeout to AuthGate

src/components/AuthGate.tsx

@@ -1,7 +1,14 @@
-import { useEffect, useState } from 'react'
+import { useEffect, useRef, useState } from 'react'
 import { useNavigate, useLocation } from 'react-router-dom'
 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')
+}
+
+
 interface Props { children: (user: User) => React.ReactNode }
 
 export function AuthGate({ children }: Props) {