* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    background-color: #444;
    font-family: Arial, sans-serif;
    height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
    overflow: hidden;
    position: relative;
    color: #fff;
}

/* mobile first: мелкая сетка и немного меньше плитки */
.grid {
    position: absolute;
    inset: 0;
    display: grid;
    grid-template-columns: repeat(auto-fill, 48px);
    grid-auto-rows: 48px;
    justify-content: center;
    align-content: center;
}

.cell {
    width: 48px;
    height: 48px;
    background-color: #444;
    background-image: url("../img/image-64x64.png");
    background-size: cover;
    background-position: center;
    opacity: 0;
    transition: opacity 0.2s ease;
}

.cell:hover {
    opacity: 1;
}

/* mobile first — компактная кнопка */
.center-btn {
    position: fixed;
    z-index: 1;
    padding: 18px 40px;
    border-radius: 999px;
    border: none;
    cursor: pointer;
    font-size: 18px;
    font-weight: 600;
    letter-spacing: 0.4px;
    color: #fff;
    background: linear-gradient(135deg, #27a5e7, #1d8ccd);
    box-shadow: 0 10px 25px rgba(0, 0, 0, 0.5);
    text-transform: uppercase;
    text-decoration: none;
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: 10px;
    transition: transform 0.15s ease, box-shadow 0.15s ease, filter 0.15s ease;
}

.center-btn:hover {
    transform: translateY(-2px) scale(1.01);
    box-shadow: 0 16px 40px rgba(0, 0, 0, 0.6);
    filter: brightness(1.05);
}

.center-btn:active {
    transform: translateY(1px) scale(0.99);
    box-shadow: 0 10px 26px rgba(0, 0, 0, 0.45);
}

/* иконка поменьше на мобилках */
.center-btn__icon {
    width: 40px;
    height: 40px;
    display: block;
}

/* планшеты ≥ 600px */
@media (min-width: 600px) {
    .grid {
        grid-template-columns: repeat(auto-fill, 56px);
        grid-auto-rows: 56px;
    }

    .cell {
        width: 56px;
        height: 56px;
    }

    .center-btn {
        padding: 22px 55px;
        font-size: 22px;
        gap: 12px;
    }

    .center-btn__icon {
        width: 48px;
        height: 48px;
    }
}

/* десктопы ≥ 1024px — твой изначальный размер */
@media (min-width: 1024px) {
    .grid {
        grid-template-columns: repeat(auto-fill, 64px);
        grid-auto-rows: 64px;
    }

    .cell {
        width: 64px;
        height: 64px;
    }

    .center-btn {
        padding: 28px 70px;
        font-size: 26px;
        font-weight: 700;
        letter-spacing: 0.6px;
        gap: 14px;
        box-shadow: 0 16px 40px rgba(0, 0, 0, 0.5);
    }

    .center-btn__icon {
        width: 56px;
        height: 56px;
    }
}