You can pass in a 4th parameter to get the absolute or relative url. By default this parameter is set to true. If you set it to false it will only return the url without the domain. You can then prefix it with the url yourself. You can for example get this value from the APP_URL or from some other config you're using ;)
$url = Url::temporarySignedRoute('viewProof', now()->addWeek(), ['proof' => $proof->id], false);