async function login() {
    const user = document.getElementById('user').value;
    const pass = document.getElementById('pass').value;
    
    const res = await fetch('/api/auth', {
        method: 'POST',
        headers: { 'Content-Type': 'application/json' },
        body: JSON.stringify({ user, pass })
    });
    
    const data = await res.json();
    
    if (res.ok) {
        // CRITICAL: This tells app.html who you are
        localStorage.setItem('logged_user', user); 
        window.location.href = '/app.html';
    } else {
        // DEBUG OUTPUT
        document.getElementById('msg').innerText = 
            `DEBUG: ${data.error} | Expected: ${data.expected?.substring(0,8)}...`;
    }
}