/* =========================================================
   THE PHYSICS BEYOND
   Global Design System — V1
   ========================================================= */


/* ---------------------------------------------------------
   1. DESIGN TOKENS
   --------------------------------------------------------- */

:root {
    /* Backgrounds */
    --bg-primary: #070B14;
    --bg-secondary: #0D1424;
    --bg-tertiary: #111A2D;

    /* Text */
    --text-primary: #F5F7FA;
    --text-secondary: #AAB3C2;
    --text-muted: #7F8999;

    /* Brand accents */
    --accent-blue: #4DA3FF;
    --accent-violet: #8B7CFF;
    --accent-gold: #E8B65A;

    /* Borders */
    --border-subtle: rgba(255, 255, 255, 0.08);
    --border-accent: rgba(77, 163, 255, 0.35);

    /* Shadows */
    --shadow-soft: 0 10px 40px rgba(0, 0, 0, 0.25);
    --shadow-glow: 0 0 40px rgba(77, 163, 255, 0.12);

    /* Layout */
    --container-width: 1180px;
    --nav-height: 76px;

    /* Typography */
    --font-heading: "Space Grotesk", "Inter", sans-serif;
    --font-body: "Inter", system-ui, -apple-system, BlinkMacSystemFont,
        "Segoe UI", sans-serif;

    /* Radius */
    --radius-sm: 6px;
    --radius-md: 12px;
    --radius-lg: 20px;

    /* Transitions */
    --transition-fast: 180ms ease;
    --transition-normal: 300ms ease;
}


/* ---------------------------------------------------------
   2. RESET
   --------------------------------------------------------- */

*,
*::before,
*::after {
    box-sizing: border-box;
}

html {
    scroll-behavior: smooth;
}

body {
    margin: 0;
    padding: 0;

    background: var(--bg-primary);
    color: var(--text-primary);

    font-family: var(--font-body);
    font-size: 16px;
    line-height: 1.7;

    -webkit-font-smoothing: antialiased;
    text-rendering: optimizeLegibility;
}

img,
svg {
    display: block;
    max-width: 100%;
}

button,
input,
textarea,
select {
    font: inherit;
}

button {
    cursor: pointer;
}

a {
    color: inherit;
    text-decoration: none;
}

p {
    margin-top: 0;
}

ul,
ol {
    margin-top: 0;
}


/* ---------------------------------------------------------
   3. TYPOGRAPHY
   --------------------------------------------------------- */

h1,
h2,
h3,
h4,
h5,
h6 {
    margin-top: 0;

    font-family: var(--font-heading);
    font-weight: 700;
    line-height: 1.15;

    color: var(--text-primary);

    letter-spacing: -0.02em;
}

h1 {
    font-size: clamp(2.35rem, 5.2vw, 4.6rem);
}

h2 {
    font-size: clamp(1.85rem, 3.2vw, 2.8rem);
}

h3 {
    font-size: clamp(1.25rem, 1.8vw, 1.6rem);
}

h4 {
    font-size: 1.15rem;
}

p {
    color: var(--text-secondary);
}

.text-muted {
    color: var(--text-muted);
}

.text-center {
    text-align: center;
}


/* ---------------------------------------------------------
   4. LAYOUT
   --------------------------------------------------------- */

.container {
    width: min(
        calc(100% - 40px),
        var(--container-width)
    );

    margin-inline: auto;
}

.section {
    padding: 88px 0;
}

.section-header {
    max-width: 760px;
    margin-bottom: 50px;
}

.section-header p {
    max-width: 680px;
    font-size: 1.08rem;
}

.eyebrow {
    display: inline-block;
    margin: 0 0 0.85rem;
    color: var(--accent-gold);
    font-size: 0.75rem;
    font-weight: 700;
    letter-spacing: 0.14em;
    line-height: 1.4;
    text-transform: uppercase;
}

.section-heading {
    max-width: 760px;
    margin-bottom: 2rem;
}

.section-heading h2 {
    max-width: 760px;
    margin-bottom: 0.75rem;
}

.section-heading > p:not(.eyebrow) {
    max-width: 680px;
    margin-bottom: 0;
    font-size: 1.05rem;
    line-height: 1.7;
}

.section-heading + .highlight,
.section-heading + .hero-actions {
    margin-top: 2rem;
}

.hero-text {
    max-width: 760px;
    margin: 0 0 0.75rem;
    color: var(--text-primary);
    font-size: clamp(1.2rem, 1.8vw, 1.45rem);
    line-height: 1.55;
}

.text-link {
    display: inline-flex;
    align-items: center;
    gap: 0.35rem;
    color: var(--accent-blue);
    font-size: 0.95rem;
    font-weight: 600;
    line-height: 1.45;
    text-decoration: none;
    transition: color var(--transition-fast), text-decoration-color var(--transition-fast);
}

.card-status {
    display: block;
    margin-top: 0.35rem;
    color: var(--text-muted);
    font-size: 0.82rem;
    font-weight: 600;
    line-height: 1.45;
}

a.text-link:hover {
    color: var(--accent-gold);
    text-decoration: underline;
    text-underline-offset: 0.2em;
}

.about-content {
    max-width: 820px;
}

.about-content > div > p {
    margin-bottom: 1rem;
    font-size: 1.05rem;
    line-height: 1.8;
}

.article-lead {
    color: var(--text-primary);
    font-size: 1.2rem;
    line-height: 1.7;
}

.about-content > div > p.article-lead {
    font-size: 1.2rem;
}


/* ---------------------------------------------------------
   5. HEADER / NAVIGATION
   --------------------------------------------------------- */

.site-header {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;

    z-index: 1000;

    height: var(--nav-height);

    background: rgba(7, 11, 20, 0.82);

    border-bottom: 1px solid var(--border-subtle);

    backdrop-filter: blur(16px);
    -webkit-backdrop-filter: blur(16px);
}

.nav-container {
    height: 100%;

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

    gap: 30px;
}

.site-logo {
    display: flex;
    align-items: center;
    text-decoration: none;
}

.site-logo img {
    display: block;
    width: 90px;
    height: auto;
}

.site-logo span:last-child {
    color: var(--accent-gold);
}

.nav-links {
    display: flex;
    align-items: center;
    gap: 28px;
}

.nav-links a {
    position: relative;

    color: var(--text-secondary);

    font-size: 0.92rem;
    font-weight: 500;

    transition:
        color var(--transition-fast),
        transform var(--transition-fast);
}

.nav-links a::after {
    content: "";

    position: absolute;

    left: 0;
    bottom: -8px;

    width: 100%;
    height: 1px;

    background: var(--accent-blue);

    transform: scaleX(0);
    transform-origin: center;

    transition: transform var(--transition-fast);
}

.nav-links a:hover,
.nav-links a.active {
    color: var(--text-primary);
}

.nav-links a:hover::after,
.nav-links a.active::after {
    transform: scaleX(1);
}

.mobile-toggle {
    display: none;

    border: 0;
    background: transparent;

    color: var(--text-primary);

    font-size: 1.5rem;
}


/* ---------------------------------------------------------
   6. HERO
   --------------------------------------------------------- */

.hero {
    position: relative;

    min-height: 100vh;

    display: flex;
    align-items: center;

    padding-top: var(--nav-height);

    overflow: hidden;

    background:
        radial-gradient(
            circle at 75% 30%,
            rgba(77, 163, 255, 0.12),
            transparent 35%
        ),
        radial-gradient(
            circle at 25% 70%,
            rgba(139, 124, 255, 0.08),
            transparent 30%
        ),
        var(--bg-primary);
}

.home-hero .container {
    position: relative;
    z-index: 1;

    display: grid;
    grid-template-columns: minmax(0, 1.3fr) minmax(300px, 0.7fr);
    align-items: center;
    gap: clamp(1.5rem, 4vw, 3.5rem);
}

.hero::before {
    content: "";

    position: absolute;
    inset: 0;

    pointer-events: none;

    background-image:
        radial-gradient(
            circle,
            rgba(255, 255, 255, 0.5) 1px,
            transparent 1px
        );

    background-size: 80px 80px;

    opacity: 0.08;
}

.hero-content {
    position: relative;
    z-index: 1;

    max-width: 900px;
}

.home-hero .hero-content {
    max-width: 760px;
}

.hero-visual {
    position: relative;
    display: grid;
    place-items: center;
    margin: 0;
}

.hero-visual::before {
    content: "";
    position: absolute;
    inset: 12%;
    border-radius: 50%;
    background: radial-gradient(circle, rgba(77, 163, 255, 0.16), transparent 68%);
    filter: blur(18px);
}

.hero-visual img {
    position: relative;
    width: min(100%, 500px);
    height: auto;
    filter: drop-shadow(0 16px 42px rgba(0, 0, 0, 0.28));
}

.hero-eyebrow {
    display: inline-block;

    margin-bottom: 22px;

    color: var(--accent-blue);

    font-size: 0.8rem;
    font-weight: 700;

    letter-spacing: 0.18em;
    text-transform: uppercase;
}

.hero h1 {
    max-width: 950px;

    margin-bottom: 28px;
}

.hero h1 span {
    color: var(--accent-gold);
}

.hero-description {
    max-width: 700px;

    margin-bottom: 40px;

    font-size: clamp(1.05rem, 2vw, 1.25rem);
}


/* ---------------------------------------------------------
   7. BUTTONS
   --------------------------------------------------------- */

.button-group,
.hero-actions {
    display: flex;
    flex-wrap: wrap;

    align-items: center;
    gap: 0.85rem;
}

.hero-actions {
    margin-top: 1.35rem;
}

.btn,
.button {
    display: inline-flex;

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

    min-height: 48px;

    padding: 0 22px;

    border-radius: var(--radius-sm);

    font-size: 0.92rem;
    font-weight: 600;

    transition:
        transform var(--transition-fast),
        background var(--transition-fast),
        border-color var(--transition-fast),
        box-shadow var(--transition-fast);
}

.btn:hover,
.button:hover {
    transform: translateY(-2px);
}

.btn-primary,
.button-primary {
    background: var(--accent-blue);
    color: #06101E;

    box-shadow:
        0 8px 30px rgba(77, 163, 255, 0.2);
}

.btn-primary:hover,
.button-primary:hover {
    background: #69B2FF;

    box-shadow:
        0 12px 36px rgba(77, 163, 255, 0.3);
}

.btn-secondary,
.button-secondary {
    border: 1px solid var(--border-subtle);

    background: rgba(255, 255, 255, 0.03);

    color: var(--text-primary);
}

.btn-secondary:hover,
.button-secondary:hover {
    border-color: var(--border-accent);

    background: rgba(77, 163, 255, 0.06);
}


/* ---------------------------------------------------------
   8. CARDS
   --------------------------------------------------------- */

.grid {
    display: grid;
    gap: 24px;
}

.grid-2 {
    grid-template-columns: repeat(2, minmax(0, 1fr));
}

.grid-3 {
    grid-template-columns: repeat(3, minmax(0, 1fr));
}

.grid-4 {
    grid-template-columns: repeat(4, minmax(0, 1fr));
}

.card {
    height: 100%;

    padding: 30px;

    border: 1px solid var(--border-subtle);
    border-radius: var(--radius-md);

    background:
        linear-gradient(
            145deg,
            rgba(255, 255, 255, 0.035),
            rgba(255, 255, 255, 0.012)
        );

    box-shadow: var(--shadow-soft);

    transition:
        transform var(--transition-normal),
        border-color var(--transition-normal),
        box-shadow var(--transition-normal);
}

.card:hover {
    transform: translateY(-5px);

    border-color: var(--border-accent);

    box-shadow:
        var(--shadow-soft),
        var(--shadow-glow);
}

.card-content > :last-child {
    margin-bottom: 0;
}

.card h3 {
    margin-bottom: 12px;
}

.card h3 a:hover {
    color: var(--accent-blue);
}

.card p {
    margin-bottom: 0;
}


/* ---------------------------------------------------------
   9. EXPLORE PHYSICS — TOPIC CARDS
   --------------------------------------------------------- */

.card-grid {
    display: grid;

    grid-template-columns:
        repeat(2, minmax(0, 1fr));

    gap: 1.5rem;

    margin-top: 2rem;
}

.article-grid {
    display: grid;
    grid-template-columns: repeat(2, minmax(0, 1fr));
    gap: 1.5rem;
    margin-top: 2rem;
}

.article-card {
    padding: 1.75rem;
    border: 1px solid var(--border-subtle);
    border-radius: var(--radius-lg);
    background: var(--bg-secondary);
}

.article-card h3 {
    margin-bottom: 0.75rem;
}

.article-card p:last-child {
    margin-bottom: 0;
}

.topic-card {
    position: relative;
    display: flex;
    flex-direction: column;
    align-items: flex-start;

    padding: 2rem;

    background: var(--bg-secondary);

    border: 1px solid var(--border-subtle);
    border-radius: var(--radius-lg);

    transition:
        transform var(--transition-normal),
        border-color var(--transition-normal),
        background var(--transition-normal);
}

.topic-card:hover {
    transform: translateY(-4px);

    border-color: rgba(77, 163, 255, 0.45);

    background: var(--bg-tertiary);
}

.topic-card .card-number {
    display: block;

    margin-bottom: 1.25rem;

    color: var(--accent-gold);

    font-family: var(--font-heading);
    font-size: 0.85rem;
    font-weight: 700;

    letter-spacing: 0.08em;
}

.topic-card h3 {
    margin-bottom: 0.75rem;

    font-size: 1.35rem;
}

.topic-card p {
    margin-bottom: 0.8rem;

    color: var(--text-secondary);

    line-height: 1.7;
}

.topic-card .text-link {
    display: inline-block;

    color: var(--accent-blue);

    font-weight: 600;

    text-decoration: none;

    transition: color var(--transition-fast);
}

.topic-card .text-link:hover {
    color: var(--accent-gold);
    text-decoration: underline;
    text-underline-offset: 0.2em;
}

.book-listing-cover {
    display: block;
    width: 100%;
    height: 360px;
    margin: 0 auto 1.5rem;
    object-fit: contain;
    border-radius: var(--radius-sm);
    background: var(--bg-primary);
}

.book-listing-price {
    color: var(--text-primary) !important;
    font-family: var(--font-heading);
    font-size: 1.2rem;
    font-weight: 700;
}


/* ---------------------------------------------------------
   10. TOPIC / CATEGORY TAGS
   --------------------------------------------------------- */

.category-tag {
    display: inline-flex;

    margin-bottom: 18px;

    color: var(--accent-gold);

    font-size: 0.72rem;
    font-weight: 700;

    letter-spacing: 0.12em;
    text-transform: uppercase;
}


/* ---------------------------------------------------------
   11. FEATURED / HIGHLIGHT AREAS
   --------------------------------------------------------- */

.highlight {
    position: relative;

    padding: 2.5rem;

    overflow: hidden;

    border: 1px solid var(--border-accent);
    border-radius: var(--radius-lg);

    background:
        radial-gradient(
            circle at top right,
            rgba(77, 163, 255, 0.12),
            transparent 35%
        ),
        var(--bg-secondary);
}

.highlight h2 {
    max-width: 760px;
}


/* =========================================================
   12. ARTICLE READING EXPERIENCE
   ========================================================= */


/* ---------------------------------------------------------
   Article header
   --------------------------------------------------------- */

.article-header {
    padding: 7rem 0 2.5rem;
}

.article-header .container {
    max-width: 900px;
}

.article-header h1 {
    max-width: 850px;

    margin-bottom: 1.75rem;

    font-size: clamp(2.5rem, 5vw, 4.25rem);

    line-height: 1.05;
}

.article-intro {
    max-width: 800px;

    margin-bottom: 1.35rem;

    color: var(--text-secondary);

    font-size: 1.12rem;

    line-height: 1.7;
}

.article-meta {
    display: flex;

    align-items: center;
    flex-wrap: wrap;

    gap: 0.6rem;

    color: var(--text-muted);

    font-size: 0.9rem;

    line-height: 1.5;
}

.article-date {
    margin-top: 0.55rem;

    color: var(--text-muted);

    font-size: 0.85rem;
}


/* ---------------------------------------------------------
   Video page player
   --------------------------------------------------------- */

.video-player-section {
    padding: 0 0 2rem;
}

.video-player {
    width: 100%;
    max-width: 1000px;
    aspect-ratio: 16 / 9;
    margin: 0 auto;
    overflow: hidden;
    border: 1px solid var(--border-subtle);
    border-radius: var(--radius-lg);
    background: #000;
    box-shadow: var(--shadow-soft);
}

.video-player iframe {
    display: block;
    width: 100%;
    height: 100%;
    border: 0;
}

.video-watch-link {
    max-width: 1000px;
    margin: 0.75rem auto 0;
    text-align: right;
}


/* ---------------------------------------------------------
   Article body layout
   --------------------------------------------------------- */
.article-body-section {
    padding: 0;
}

.article-layout {
    display: flex;

    justify-content: center;

    padding-top: 1.5rem;
    padding-bottom: 4rem;
}

.article-content {
    width: 100%;

    max-width: 780px;
}


/* ---------------------------------------------------------
   Article body typography
   --------------------------------------------------------- */

.article-content p {
    margin: 0 0 1.2rem;

    color: var(--text-secondary);

    font-size: 1.05rem;

    line-height: 1.75;
}

.article-content .article-lead {
    margin: 0 0 2.4rem;

    color: var(--text-primary);

    font-size: 1.15rem;

    line-height: 1.75;
}

.article-content h2 {
    margin: 3rem 0 1rem;

    color: var(--text-primary);

    font-size: 2rem;

    line-height: 1.2;
}

.article-content h3 {
    margin: 2.25rem 0 0.85rem;

    color: var(--text-primary);

    font-size: 1.45rem;

    line-height: 1.25;
}

.article-content strong {
    color: var(--text-primary);
}


/* ---------------------------------------------------------
   Article lists
   --------------------------------------------------------- */

.article-content ul,
.article-content ol {
    margin: 0 0 1.5rem;

    padding-left: 1.4rem;
}

.article-content li {
    margin-bottom: 0.65rem;

    color: var(--text-secondary);

    font-size: 1.05rem;

    line-height: 1.7;
}


/* ---------------------------------------------------------
   Article images
   --------------------------------------------------------- */

.article-content img {
    width: 100%;

    margin: 2.25rem 0;

    border-radius: var(--radius-md);
}


/* ---------------------------------------------------------
   Article callout / key idea
   --------------------------------------------------------- */

.article-note {
    margin: 2.75rem 0;

    padding: 1.75rem 2rem;

    background: var(--bg-secondary);

    border: 1px solid var(--border-accent);
    border-left: 3px solid var(--accent-blue);

    border-radius: var(--radius-lg);
}

.article-note .eyebrow {
    margin-bottom: 0.85rem;

    color: var(--accent-gold);

    font-size: 0.78rem;
    font-weight: 700;

    letter-spacing: 0.12em;

    text-transform: uppercase;
}

.article-note p:last-child {
    margin-bottom: 0;

    color: var(--text-primary);
}


/* ---------------------------------------------------------
   Article sources
   --------------------------------------------------------- */

.article-sources {
    margin: 1.25rem 0 0;

    padding-left: 1.25rem;
}

.article-sources li {
    margin-bottom: 0.65rem;

    color: var(--text-secondary);

    line-height: 1.7;
}

.article-sources a {
    color: var(--accent-blue);

    text-decoration: none;

    transition: color var(--transition-fast);
}

.article-sources a:hover {
    color: var(--accent-gold);

    text-decoration: underline;
}


/* ---------------------------------------------------------
   Article navigation
   --------------------------------------------------------- */

.article-navigation-section {
    padding: 1rem 0;
}

.article-navigation {
    display: flex;

    justify-content: space-between;
    align-items: center;

    gap: 2rem;

    padding: 2rem 0;

    border-top: 1px solid var(--border-subtle);
    border-bottom: 1px solid var(--border-subtle);
}

.article-navigation .text-link {
    color: var(--text-primary);

    font-weight: 600;

    text-decoration: none;

    transition: color var(--transition-fast);
}

.article-navigation .text-link:hover {
    color: var(--accent-blue);
}


/* ---------------------------------------------------------
   13. FORMS
   --------------------------------------------------------- */

.form-group {
    margin-bottom: 20px;
}

.form-label {
    display: block;

    margin-bottom: 8px;

    color: var(--text-primary);

    font-size: 0.9rem;
    font-weight: 600;
}

.form-input,
.form-textarea {
    width: 100%;

    border: 1px solid var(--border-subtle);
    border-radius: var(--radius-sm);

    padding: 13px 15px;

    background: var(--bg-secondary);
    color: var(--text-primary);

    outline: none;

    transition:
        border-color var(--transition-fast),
        box-shadow var(--transition-fast);
}

.form-textarea {
    min-height: 160px;

    resize: vertical;
}

.form-input:focus,
.form-textarea:focus {
    border-color: var(--accent-blue);

    box-shadow:
        0 0 0 3px rgba(77, 163, 255, 0.1);
}


/* ---------------------------------------------------------
   14. FOOTER
   --------------------------------------------------------- */

.site-footer {
    padding: 60px 0 30px;

    border-top: 1px solid var(--border-subtle);

    background: #050810;
}

.footer-grid {
    display: grid;

    grid-template-columns:
        2fr repeat(3, 1fr);

    gap: 50px;

    padding-bottom: 45px;
}

/* ---------------------------------------------------------
   Footer brand alignment
   --------------------------------------------------------- */

.footer-grid > div:first-child {
    display: flex;
    flex-direction: column;
    align-items: flex-start;
}

.footer-grid > div:first-child .site-logo {
    margin-left: -10px;
}

.footer-grid > div:first-child p {
    margin-top: 14px;
    margin-bottom: 0;
    max-width: 380px;
}

.footer-brand p {
    max-width: 380px;
}

.footer-column h4 {
    margin-bottom: 18px;

    font-size: 0.9rem;

    color: var(--text-primary);
}

.footer-links {
    display: grid;

    gap: 10px;

    list-style: none;

    padding: 0;
}

.footer-links a {
    color: var(--text-muted);

    font-size: 0.9rem;

    transition: color var(--transition-fast);
}

.footer-links a:hover {
    color: var(--text-primary);
}

/* ---------------------------------------------------------
   Clean URL footer links
   --------------------------------------------------------- */

.footer-grid > div:not(.footer-brand) > h3 {
    margin-bottom: 18px;

    font-size: 0.9rem;
    color: var(--text-primary);
}

.footer-grid > div:not(.footer-brand) > a {
    display: block;

    margin-bottom: 10px;

    color: var(--text-muted);

    font-size: 0.9rem;

    transition: color var(--transition-fast);
}

.footer-grid > div:not(.footer-brand) > a:hover {
    color: var(--text-primary);
}

.footer-bottom {
    display: flex;

    align-items: center;
    justify-content: space-between;

    gap: 20px;

    padding-top: 25px;

    border-top: 1px solid var(--border-subtle);
}

.footer-bottom p {
    margin: 0;

    font-size: 0.8rem;
}


/* ---------------------------------------------------------
   15. FOCUS STATES
   --------------------------------------------------------- */

:focus-visible {
    outline: 2px solid var(--accent-blue);

    outline-offset: 4px;
}


/* ---------------------------------------------------------
   16. SIMPLE SCROLL ANIMATION
   --------------------------------------------------------- */

.fade-in {
    opacity: 0;

    transform: translateY(18px);

    transition:
        opacity 700ms ease,
        transform 700ms ease;
}

.fade-in.visible {
    opacity: 1;

    transform: translateY(0);
}


/*
   Article content must remain visible even if the
   scroll-animation observer does not fire.
*/

.article-content,
.article-content.fade-in {
    opacity: 1;

    transform: none;
}


/* ---------------------------------------------------------
   17. RESPONSIVE — TABLET
   --------------------------------------------------------- */

@media (max-width: 900px) {

    .home-hero .container {
        grid-template-columns: 1fr;
        gap: 1.5rem;
    }

    .hero-visual img {
        width: min(65vw, 360px);
    }

    .grid-3,
    .grid-4 {
        grid-template-columns:
            repeat(2, minmax(0, 1fr));
    }

    .footer-grid {
        grid-template-columns:
            repeat(2, 1fr);
    }
}


/* ---------------------------------------------------------
   18. RESPONSIVE — MOBILE
   --------------------------------------------------------- */

@media (max-width: 720px) {

    :root {
        --nav-height: 68px;
    }


    /* Layout */

    .container {
        width: min(
            calc(100% - 28px),
            var(--container-width)
        );
    }

    .section {
        padding: 72px 0;
    }

    .hero-text {
        font-size: 1.15rem;
    }

    .section-heading {
        margin-bottom: 1.5rem;
    }

    .section-heading > p:not(.eyebrow) {
        font-size: 1rem;
    }


    /* Navigation */

    .mobile-toggle {
        display: block;
    }

    .nav-links {
        position: absolute;

        top: var(--nav-height);
        left: 0;
        right: 0;

        display: none;

        flex-direction: column;
        align-items: stretch;

        gap: 0;

        padding: 12px 20px 20px;

        border-bottom: 1px solid var(--border-subtle);

        background: rgba(7, 11, 20, 0.98);
    }

    .nav-links.active {
        display: flex;
    }

    .nav-links a {
        padding: 14px 0;
    }

    .nav-links a::after {
        display: none;
    }


    /* Grids */

    .grid-2,
    .grid-3,
    .grid-4,
    .card-grid,
    .article-grid {
        grid-template-columns: 1fr;
    }


    /* Highlight */

    .highlight {
        padding: 30px 24px;
    }


    /* Footer */

    .footer-grid {
        grid-template-columns: 1fr;

        gap: 35px;
    }

    .footer-bottom {
        flex-direction: column;

        align-items: flex-start;
    }


    /* Buttons */

    .button-group,
    .hero-actions {
        flex-direction: column;

        align-items: stretch;
    }

    .btn,
    .button {
        width: 100%;
    }


    /* Topic cards */

    .topic-card {
        padding: 1.5rem;
    }


    /* Article header */

    .article-header {
        padding: 5rem 0 2rem;
    }

    .article-header h1 {
        margin-bottom: 1.5rem;

        font-size: 2.6rem;

        line-height: 1.08;
    }

    .article-intro {
        margin-bottom: 1.2rem;

        font-size: 1.05rem;

        line-height: 1.7;
    }

    .article-meta {
        font-size: 0.85rem;
    }

    .article-date {
        font-size: 0.8rem;
    }


    /* Article body */

    .article-layout {
        padding-top: 1rem;

        padding-bottom: 3rem;
    }

    .article-content p {
        margin-bottom: 1.1rem;

        font-size: 1rem;

        line-height: 1.7;
    }

    .article-content .article-lead {
        margin-bottom: 2rem;

        font-size: 1.08rem;

        line-height: 1.7;
    }

    .article-content h2 {
        margin-top: 2.5rem;
        margin-bottom: 0.9rem;

        font-size: 1.7rem;

        line-height: 1.2;
    }

    .article-content h3 {
        margin-top: 2rem;
        margin-bottom: 0.75rem;

        font-size: 1.3rem;
    }

    .article-content li {
        font-size: 1rem;

        line-height: 1.7;
    }


    /* Article callout */

    .article-note {
        margin: 2.25rem 0;

        padding: 1.5rem;
    }


    /* Article navigation */

    .article-navigation {
        flex-direction: column;

        align-items: flex-start;

        gap: 1rem;

        padding: 1.5rem 0;
    }
}


/* ---------------------------------------------------------
   19. REDUCED MOTION
   --------------------------------------------------------- */

@media (prefers-reduced-motion: reduce) {

    html {
        scroll-behavior: auto;
    }

    *,
    *::before,
    *::after {
        transition-duration: 0.01ms !important;

        animation-duration: 0.01ms !important;

        animation-iteration-count: 1 !important;
    }

    .fade-in {
        opacity: 1;

        transform: none;
    }
}

/* =========================================
   MOBILE HEADER
========================================= */

@media (max-width: 720px) {

    .site-header {
        height: auto;
        min-height: 0;
    }

    .nav-container {
        flex-direction: column;
        align-items: flex-start;
        gap: 0.75rem;
        padding-top: 0.85rem;
        padding-bottom: 0.85rem;
    }

    .site-nav {
        width: 100%;
        display: flex;
        flex-wrap: wrap;
        justify-content: flex-start;
        align-items: center;
        column-gap: 0.9rem;
        row-gap: 0.5rem;
        padding-bottom: 0;
    }

    .site-nav a {
        font-size: 0.9rem;
        line-height: 1.4;
    }

        .hero {
        padding-top: 8rem;
    }

}

/* =========================================
   BOOKSHELF PRODUCT PAGES
========================================= */

.product-breadcrumb-section {
    padding: 6rem 0 0;
}

.product-breadcrumb {
    display: flex;
    align-items: center;
    gap: 0.65rem;
    color: var(--text-muted);
    font-size: 0.9rem;
}

.product-breadcrumb a {
    color: var(--accent-blue);
    text-decoration: none;
    transition: color var(--transition-fast);
}

.product-breadcrumb a:hover {
    color: var(--accent-gold);
}

.product-layout {
    display: grid;
    grid-template-columns: minmax(280px, 0.85fr) minmax(0, 1.15fr);
    gap: 4rem;
    align-items: start;
}

.product-media {
    width: 100%;
    margin: 0;
}

.product-cover-image {
    display: block;
    width: 100%;
    max-height: 760px;
    object-fit: contain;
    border: 1px solid var(--border-subtle);
    border-radius: var(--radius-lg);
    background: var(--bg-secondary);
}

.product-cover-caption {
    margin-top: 0.75rem;
    color: var(--text-muted);
    font-size: 0.8rem;
    text-align: center;
}

.product-cover-placeholder {
    min-height: 520px;
    padding: 2rem;
    border: 1px solid rgba(77, 163, 255, 0.25);
    border-radius: var(--radius-lg);
    background:
        radial-gradient(
            circle at 70% 20%,
            rgba(139, 124, 255, 0.12),
            transparent 45%
        ),
        var(--bg-secondary);
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    text-align: center;
}

.product-cover-placeholder span {
    color: var(--accent-gold);
    font-size: 0.75rem;
    font-weight: 700;
    letter-spacing: 0.14em;
}

.product-cover-placeholder strong {
    margin-top: 0.5rem;
    color: var(--text-primary);
    font-family: var(--font-heading);
    font-size: 2.5rem;
    line-height: 1;
}

.product-cover-placeholder small {
    margin-top: 1rem;
    color: var(--text-muted);
    font-size: 0.85rem;
}

.product-info {
    padding-top: 0.5rem;
}

.product-brand {
    margin: 1.25rem 0 0.65rem;
    color: var(--accent-blue);
    font-size: 0.9rem;
    font-weight: 600;
    letter-spacing: 0.04em;
    text-transform: uppercase;
}

.product-info h1 {
    margin: 0;
    color: var(--text-primary);
    font-family: var(--font-heading);
    font-size: clamp(2.4rem, 5vw, 4.25rem);
    line-height: 1.05;
}

.product-lead {
    margin: 1.5rem 0 0;
    color: var(--text-secondary);
    font-size: 1.15rem;
    line-height: 1.75;
}

.product-meta {
    display: grid;
    grid-template-columns: repeat(2, minmax(0, 1fr));
    gap: 1rem;
    margin-top: 2rem;
}

.product-meta > div {
    padding: 1rem;
    border: 1px solid rgba(170, 179, 194, 0.12);
    border-radius: var(--radius-md);
    background: var(--bg-secondary);
}

.product-meta-label {
    display: block;
    margin-bottom: 0.35rem;
    color: var(--text-muted);
    font-size: 0.78rem;
    text-transform: uppercase;
    letter-spacing: 0.08em;
}

.product-meta > div > span:last-child {
    color: var(--text-primary);
    font-size: 0.95rem;
}

.product-purchase {
    margin-top: 2rem;
    padding: 1.5rem;
    border: 1px solid rgba(77, 163, 255, 0.22);
    border-radius: var(--radius-md);
    background: var(--bg-secondary);
}

.product-price {
    color: var(--text-primary);
    font-family: var(--font-heading);
    font-size: 2rem;
    font-weight: 700;
}

.product-availability {
    display: block;
    margin-top: 0.35rem;
    color: var(--text-muted);
    font-size: 0.85rem;
}

.product-purchase .btn {
    margin-top: 1.25rem;
}

.product-purchase .btn:disabled {
    opacity: 0.55;
    cursor: not-allowed;
}

.purchase-note {
    margin: 0.85rem 0 0;
    color: var(--text-muted);
    font-size: 0.8rem;
    line-height: 1.5;
}

.product-content {
    max-width: 820px;
}

.product-content > p {
    margin: 0 0 1.2rem;
    color: var(--text-secondary);
    font-size: 1.05rem;
    line-height: 1.75;
}

.product-preview-placeholder {
    min-height: 260px;
    padding: 3rem;
    border: 1px dashed rgba(77, 163, 255, 0.3);
    border-radius: var(--radius-lg);
    background: var(--bg-secondary);
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    text-align: center;
}

.product-preview-placeholder span {
    color: var(--accent-gold);
    font-size: 0.8rem;
    font-weight: 700;
    letter-spacing: 0.14em;
}

.product-preview-placeholder p {
    margin: 0.75rem 0 0;
    color: var(--text-muted);
}

@media (max-width: 900px) {

    .product-layout {
        grid-template-columns: 1fr;
        gap: 2.5rem;
    }

    .product-cover-placeholder {
        min-height: 420px;
        max-width: 520px;
        margin: 0 auto;
    }

}

@media (max-width: 720px) {

    .product-breadcrumb-section {
        padding-top: 8rem 0 0;
    }

    .product-layout {
        gap: 2rem;
    }

    .product-cover-placeholder {
        min-height: 360px;
    }

    .product-info h1 {
        font-size: 2.5rem;
    }

    .product-lead {
        font-size: 1.05rem;
    }

    .product-meta {
        grid-template-columns: 1fr;
    }

    .product-preview-placeholder {
        min-height: 200px;
        padding: 2rem;
    }

}