fix: remove stale useRouter from LoginModal

frontend/src/components/LoginModal.tsx

@@ -4,7 +4,6 @@ import { useState } from 'react'
 import { useAuth } from '@/hooks/useAuth'
 
 export default function LoginModal({ onClose }: { onClose: () => void }) {
-  const router = useRouter()
   const { login } = useAuth()
   const [email, setEmail] = useState('')
   const [password, setPassword] = useState('')