.backtop{
    position: fixed;
    right: 16px;
    bottom: 16px;
    z-index: 9999;
    width: 44px;
    height: 44px;
    display: inline-flex;
    align-items: center;
    justify-content: center;
    border: none;
    border-radius: 999px;
    background: var(--primary, #0ea5e9);
    color: #fff;
    box-shadow: 0 10px 20px rgba(0,0,0,.15);
    cursor: pointer;

    opacity: 0;
    transform: translateY(8px);
    pointer-events: none;
    transition: opacity .2s ease, transform .2s ease, background .2s ease;
}
.backtop:hover{ background: #000d13; }
.backtop:focus-visible{ outline: 2px solid #fff; outline-offset: 2px; }

.backtop.show{
    opacity: 1;
    transform: translateY(0);
    pointer-events: auto;
}

@media (prefers-reduced-motion: reduce){
    .backtop{ transition: none; }
}