/* ==========================================================================
   BACK-TO-TOP — Scroll-to-top button
   ========================================================================== */

.back-to-top {
    position: fixed;
    bottom: var(--space-6, 1.5rem);
    right: var(--space-6, 1.5rem);
    width: 44px;
    height: 44px;
    border-radius: var(--rounded-full, 9999px);
    background-color: var(--color-charcoal, #36454f);
    color: var(--text-inverse, #ffffff);
    border: none;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    box-shadow: var(--shadow-lg, 0 10px 15px rgba(0, 0, 0, 0.1));
    opacity: 0;
    visibility: hidden;
    transform: translateY(10px);
    transition: opacity var(--transition-base, 300ms ease),
                visibility var(--transition-base, 300ms ease),
                transform var(--transition-base, 300ms ease),
                background-color var(--transition-fast, 150ms ease);
    z-index: var(--z-sticky, 1020);
}

.back-to-top.is-visible {
    opacity: 1;
    visibility: visible;
    transform: translateY(0);
}

.back-to-top:hover {
    background-color: var(--color-accent, #4a90a4);
}