Add button to install the PWA as a standalone app

[terremoth]

Something similar to:

    let deferredPrompt;
    const installButton = document.getElementById('installButton');

    window.addEventListener('beforeinstallprompt', (e) => {
      e.preventDefault();
      deferredPrompt = e;
      installButton.style.display = 'block';
    });

    installButton.addEventListener('click', async () => {
      installButton.style.display = 'none';
      deferredPrompt.prompt();
      const { outcome } = await deferredPrompt.userChoice;
      console.log(`User ${outcome} the installation`);
      deferredPrompt = null;
    });

    // Opcional: evento para quando a app for instalada
    window.addEventListener('appinstalled', () => {
        console.log('App successfully installed!');
    });