* {
    box-sizing: border-box;
}

html,
body {
    margin: 0;
    padding: 0;
    min-height: 100%;
}

body {
    font-family: Arial, Helvetica, sans-serif;
    background:
        radial-gradient(circle at top, #ffffff 0%, #f4f4f4 55%, #e9e9e9 100%);
    color: #222;
}

.catalog-shell {
    width: 100%;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
    align-items: center;
    padding: 18px 16px 22px;
}

.catalog-header {
    text-align: center;
    margin-bottom: 12px;
}

.brand-title {
    font-size: 20px;
    letter-spacing: 3px;
    font-weight: 700;
}

.catalog-subtitle {
    margin-top: 3px;
    font-size: 12px;
    letter-spacing: 2px;
    color: #666;
    text-transform: uppercase;
}

.book-stage {
     width: 100%;
    height: calc(100vh - 190px);
    min-height: 480px;
    max-height: 720px;

    display: flex;
    align-items: center;
    justify-content: center;

    position: relative;
    overflow: hidden;
}

#book {
    margin: auto;
}

.page {
    background: white;
    overflow: hidden;
    box-shadow: inset 0 0 18px rgba(0, 0, 0, 0.05);
}

.page img {
    width: 100%;
    height: 100%;
    display: block;
    object-fit: contain;
    background: white;

    -webkit-user-drag: none;
    user-select: none;
}

.nav-button {
    position: absolute;
    top: 50%;
    transform: translateY(-50%);
    z-index: 50;

    width: 52px;
    height: 52px;

    border: 0;
    border-radius: 50%;

    background: rgba(255, 255, 255, 0.92);
    color: #222;

    font-size: 40px;
    line-height: 40px;

    cursor: pointer;

    box-shadow: 0 5px 20px rgba(0, 0, 0, 0.12);

    transition:
        transform 0.2s ease,
        background 0.2s ease,
        box-shadow 0.2s ease;
}

.nav-button:hover {
    background: white;
    box-shadow: 0 7px 24px rgba(0, 0, 0, 0.18);
}

.nav-button:active {
    transform: translateY(-50%) scale(0.94);
}

.nav-button.left {
    left: 18px;
}

.nav-button.right {
    right: 18px;
}

.catalog-controls {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 18px;

    margin-top: 12px;

    font-size: 13px;
    color: #555;
}

#fullscreenBtn {
    border: 1px solid #ccc;
    border-radius: 999px;
    padding: 8px 15px;
    background: white;
    cursor: pointer;
    font-size: 12px;
}

#fullscreenBtn:hover {
    border-color: #888;
}

@media (max-width: 900px) {

    .catalog-shell {
    width: 100%;
    min-height: 100vh;

    display: flex;
    flex-direction: column;
    align-items: center;

    padding: 12px 16px 16px;
}

    .catalog-header {
        margin-bottom: 4px;
    }

    .brand-title {
        font-size: 16px;
        letter-spacing: 2px;
    }

    .catalog-subtitle {
        font-size: 10px;
    }

    .book-stage {
        min-height: 520px;
    }

    .nav-button {
        width: 42px;
        height: 42px;
        font-size: 32px;
    }

    .nav-button.left {
        left: 4px;
    }

    .nav-button.right {
        right: 4px;
    }
}

@media (max-width: 600px) {

    .catalog-shell {
        padding-left: 0;
        padding-right: 0;
    }

    .book-stage {
        min-height: 490px;
    }

    .nav-button {
        width: 38px;
        height: 38px;
        font-size: 29px;

        background: rgba(255, 255, 255, 0.84);
    }

    .catalog-controls {
        margin-top: 8px;
    }
}