Add shared device inactivity timeout to AuthGate

frontend/src/components/AuthGate.tsx

@@ -1,6 +1,13 @@
 import { useEffect, useState, createContext, useContext } 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')
+}
+
+
 interface AuthCtx { user: User }
 const Ctx = createContext<AuthCtx | null>(null)