/* ==========================================================================
   Trang công khai render sẵn trên server (SEO): kỳ thi, đề thi, dạng toán, bài viết.
   Không dùng Tailwind CDN để tải nhanh. Cùng token "Chàm & Cam" với public/index.html.
   Mọi class có tiền tố s- để không đụng design system của SPA.
   ========================================================================== */
.s-site {
  --ink: #1B1A3A; --ink-soft: #5B6079; --brand: #5048D6; --brand-dark: #4238B8; --brand-50: #EEF0FF; --brand-100: #E0E3FF;
  --accent: #FF8A3D; --accent-dark: #F06B1C; --accent-50: #FFF4EB; --mint: #12B886; --mint-50: #E8FBF3; --coral: #F0506E; --coral-50: #FFF0F3;
  --bg: #F5F7FC; --line: #E6E9F2; --shadow: 0 1px 2px rgba(16,24,64,.04), 0 8px 24px rgba(16,24,64,.06);
}
.s-site { margin: 0; font-family: 'Lexend', system-ui, -apple-system, 'Segoe UI', sans-serif; font-size: 17px; line-height: 1.65; color: var(--ink);
  background: var(--bg) radial-gradient(circle, rgba(80,72,214,.10) 1.2px, transparent 1.3px) 0 0 / 26px 26px; -webkit-text-size-adjust: 100%; }
@media (min-width: 768px) { .s-site { font-size: 18px; } }
.s-site *, .s-site *::before, .s-site *::after { box-sizing: border-box; }
.s-site img { max-width: 100%; height: auto; }
.s-site a { color: var(--brand); text-underline-offset: 3px; }
.s-site h1, .s-site h2, .s-site h3 { line-height: 1.25; letter-spacing: -.01em; font-weight: 700; margin: 0; }
.s-site h1 { font-size: clamp(1.75rem, 4.6vw, 2.6rem); }
.s-site h2 { font-size: clamp(1.4rem, 3.2vw, 1.9rem); }
.s-site h3 { font-size: 1.15rem; }
.s-site p { margin: .6rem 0 0; }
.s-wrap { max-width: 72rem; margin: 0 auto; padding: 0 16px; }
.s-narrow { max-width: 48rem; }
.s-skip { position: absolute; left: -999px; }
.s-show-sm { display: none; }
@media (max-width: 420px) { .s-hide-sm { display: none; } .s-show-sm { display: inline; } }
.s-skip:focus { left: 16px; top: 8px; z-index: 50; background: #fff; padding: .5rem 1rem; border-radius: .5rem; }

/* Header + nav */
.s-header { position: sticky; top: 0; z-index: 20; background: rgba(255,255,255,.92); backdrop-filter: blur(8px); border-bottom: 1px solid var(--line); }
.s-header .s-wrap { display: flex; align-items: center; gap: 12px; min-height: 64px; }
.s-logo { display: flex; align-items: center; gap: 10px; text-decoration: none; color: var(--ink) !important; font-weight: 700; font-size: 1.15rem; white-space: nowrap; }
.s-logo img { width: 40px; height: 40px; border-radius: 12px; }
.s-logo b { color: var(--accent); }
.s-nav { display: flex; gap: 4px; margin-left: auto; overflow-x: auto; scrollbar-width: none; }
.s-nav::-webkit-scrollbar { display: none; }
.s-nav a { display: inline-flex; align-items: center; min-height: 44px; padding: 0 12px; border-radius: 999px; text-decoration: none; color: var(--ink-soft); font-weight: 600; font-size: .92rem; white-space: nowrap; }
.s-nav a:hover, .s-nav a[aria-current="page"] { background: var(--brand-50); color: var(--brand-dark); }
@media (max-width: 760px) { .s-header { position: static; } .s-header .s-wrap { flex-wrap: wrap; column-gap: 8px; row-gap: 0; padding-top: 8px; } .s-header .s-cta-top { margin-left: auto; padding: .5rem 1rem; } .s-nav { order: 3; width: 100%; margin: 0 -8px 4px; } .s-logo span { font-size: 1.05rem; } }

/* Buttons, chips, cards */
.s-btn { display: inline-flex; align-items: center; justify-content: center; gap: .5rem; min-height: 48px; padding: .6rem 1.35rem; border-radius: 999px;
  font-weight: 600; font-size: 1rem; text-decoration: none; border: 1.5px solid transparent; cursor: pointer; transition: transform .15s ease, background-color .15s ease; text-align: center; }
.s-btn:hover { transform: translateY(-1px); }
.s-btn-accent { background: var(--accent); color: #fff !important; box-shadow: 0 6px 18px rgba(255,138,61,.35); }
.s-btn-accent:hover { background: var(--accent-dark); }
.s-btn-primary { background: var(--brand); color: #fff !important; box-shadow: 0 6px 18px rgba(80,72,214,.28); }
.s-btn-ghost { background: #fff; color: var(--ink) !important; border-color: #CBD5E1; }
.s-btn-glass { background: rgba(255,255,255,.12); color: #fff !important; border-color: rgba(255,255,255,.35); }
.s-btn-sm { min-height: 44px; padding: .4rem 1rem; font-size: .92rem; }
.s-chip { display: inline-flex; align-items: center; gap: .3rem; padding: .25rem .75rem; border-radius: 999px; background: var(--brand-50); color: var(--brand-dark); font-weight: 600; font-size: .85rem; text-decoration: none; }
.s-chip-accent { background: var(--accent-50); color: #C85412; }
.s-chip-mint { background: var(--mint-50); color: #0B8A61; }
.s-chips { display: flex; flex-wrap: wrap; gap: 8px; margin-top: 12px; }
.s-card { background: #fff; border: 1px solid var(--line); border-radius: 1.25rem; box-shadow: var(--shadow); padding: 20px; }
a.s-card { display: block; text-decoration: none; color: var(--ink); transition: transform .18s ease, border-color .18s ease; }
a.s-card:hover { transform: translateY(-2px); border-color: #C5CAFF; }
.s-muted { color: var(--ink-soft); }
.s-eyebrow { display: inline-block; font-size: .8rem; font-weight: 700; letter-spacing: .08em; text-transform: uppercase; color: var(--brand); }
.s-grid { display: grid; gap: 16px; }
@media (min-width: 640px) { .s-grid-2 { grid-template-columns: repeat(2, 1fr); } .s-grid-3 { grid-template-columns: repeat(2, 1fr); } }
@media (min-width: 960px) { .s-grid-3 { grid-template-columns: repeat(3, 1fr); } .s-grid-4 { grid-template-columns: repeat(4, 1fr); } }
@media (min-width: 640px) and (max-width: 959px) { .s-grid-4 { grid-template-columns: repeat(2, 1fr); } }
.s-section { margin-top: 48px; }
.s-section > h2 + p { margin-top: .5rem; }
.s-icon { width: 48px; height: 48px; border-radius: 14px; background: var(--brand-50); display: flex; align-items: center; justify-content: center; font-size: 1.5rem; }

/* Hero */
.s-hero { color: #fff; border-radius: 0 0 2rem 2rem; padding: 40px 0 48px; position: relative; overflow: hidden;
  background: linear-gradient(rgba(255,255,255,.06) 1px, transparent 1px) 0 0 / 40px 40px, linear-gradient(90deg, rgba(255,255,255,.06) 1px, transparent 1px) 0 0 / 40px 40px,
    radial-gradient(120% 90% at 80% 0%, #3B36A8 0%, #2A2670 45%, #1B1A3A 100%); }
.s-hero h1 { color: #fff; }
.s-hero .s-lead { color: rgba(255,255,255,.85); font-size: 1.1rem; max-width: 46rem; }
.s-hero .s-hl { color: #FFAE72; }
.s-hero .s-actions { display: flex; flex-wrap: wrap; gap: 12px; margin-top: 24px; }
.s-hero .s-pill { display: inline-block; padding: .3rem .9rem; border-radius: 999px; background: rgba(255,255,255,.12); border: 1px solid rgba(255,255,255,.25); font-weight: 600; font-size: .9rem; }
.s-hero-sm { padding: 28px 0 32px; }
.s-hero .s-crumbs, .s-hero .s-crumbs a { color: rgba(255,255,255,.75); }
.s-facts { display: flex; flex-wrap: wrap; gap: 10px; margin-top: 18px; padding: 0; list-style: none; }
.s-facts li { background: rgba(255,255,255,.1); border: 1px solid rgba(255,255,255,.2); border-radius: 999px; padding: .3rem .85rem; font-weight: 600; font-size: .9rem; }

/* Breadcrumbs */
.s-crumbs { font-size: .9rem; margin-bottom: 12px; display: flex; flex-wrap: wrap; gap: 4px; padding: 0; list-style: none; }
.s-crumbs li + li::before { content: "›"; margin-right: 4px; opacity: .6; }
.s-crumbs a { text-decoration: none; }

/* "Kho PDF vs LuyệnToán" — khối thông điệp chính */
.s-vs { display: grid; gap: 16px; margin-top: 24px; }
@media (min-width: 760px) { .s-vs { grid-template-columns: 1fr 1.25fr; } }
.s-vs-col { border-radius: 1.25rem; padding: 20px; }
.s-vs-pdf { background: #fff; border: 1.5px dashed #CBD5E1; }
.s-vs-pdf h3 { color: var(--ink-soft); }
.s-vs-lt { background: linear-gradient(160deg, #fff 0%, #F4F5FF 100%); border: 2px solid var(--brand); box-shadow: 0 10px 30px rgba(80,72,214,.15); }
.s-vs ul { list-style: none; padding: 0; margin: 12px 0 0; display: grid; gap: 10px; }
.s-vs li { display: flex; gap: 10px; align-items: flex-start; }
.s-vs li .s-ic { flex: 0 0 28px; height: 28px; border-radius: 9px; display: flex; align-items: center; justify-content: center; font-size: .95rem; font-weight: 700; }
.s-vs-pdf .s-ic { background: #F1F3F8; color: #94A3B8; }
.s-vs-lt .s-ic { background: var(--mint-50); color: var(--mint); }
.s-vs li b { display: block; }
.s-vs li span { color: var(--ink-soft); font-size: .95rem; }
.s-table-wrap { overflow-x: auto; margin-top: 16px; border-radius: 1rem; border: 1px solid var(--line); background: #fff; }
.s-table { width: 100%; border-collapse: collapse; min-width: 520px; font-size: .95rem; }
.s-table th, .s-table td { padding: 12px 14px; text-align: left; border-bottom: 1px solid var(--line); vertical-align: top; }
.s-table th { background: #F8F9FE; font-weight: 700; }
.s-table tr:last-child td { border-bottom: 0; }
.s-yes { color: #0B8A61; font-weight: 600; }
.s-no { color: #94A3B8; }

/* Báo cáo chẩn đoán (minh họa) */
.s-diag { display: grid; gap: 10px; margin-top: 12px; }
.s-diag-row { display: grid; grid-template-columns: 9.5rem 1fr 3rem; align-items: center; gap: 10px; font-size: .92rem; font-weight: 600; }
.s-bar { height: 12px; border-radius: 999px; background: #EEF0F6; overflow: hidden; }
.s-bar i { display: block; height: 100%; border-radius: 999px; background: var(--mint); }
.s-bar i.s-weak { background: var(--coral); }
.s-bar i.s-mid { background: var(--accent); }
.s-tag-weak { display: inline-block; margin-top: 12px; padding: .5rem .8rem; border-radius: .8rem; background: var(--coral-50); color: #B4234A; font-weight: 600; font-size: .92rem; }

/* Lời giải trực quan từng bước */
.s-steps { list-style: none; padding: 0; margin: 14px 0 0; display: grid; gap: 10px; counter-reset: st; }
.s-steps li { counter-increment: st; position: relative; padding: 12px 14px 12px 52px; border-radius: 1rem; background: #F8F9FE; border: 1px solid var(--line); }
.s-steps li::before { content: counter(st); position: absolute; left: 12px; top: 12px; width: 28px; height: 28px; border-radius: 9px; background: var(--brand); color: #fff; font-weight: 700; display: flex; align-items: center; justify-content: center; font-size: .9rem; }
.s-calc { display: inline-block; margin-top: 6px; padding: .3rem .7rem; border-radius: .6rem; background: #fff; border: 1px solid var(--brand-100); font-weight: 700; color: var(--brand-dark); font-variant-numeric: tabular-nums; }
.s-answer { margin-top: 12px; padding: 12px 14px; border-radius: 1rem; background: var(--mint-50); color: #0B6B4C; font-weight: 600; }
.s-trap { margin-top: 10px; padding: 12px 14px; border-radius: 1rem; background: var(--accent-50); color: #9C4111; }
.s-tip { margin-top: 10px; padding: 12px 14px; border-radius: 1rem; background: var(--brand-50); color: var(--brand-dark); }

/* Câu hỏi xem trước */
.s-q { margin-top: 16px; }
.s-q-head { display: flex; flex-wrap: wrap; align-items: center; gap: 8px; }
.s-q-no { font-weight: 700; color: var(--brand); }
.s-q-en { color: var(--ink-soft); font-style: italic; margin-top: .4rem; }
.s-q img { display: block; margin-top: 12px; border-radius: .8rem; border: 1px solid var(--line); background: #fff; max-height: 320px; width: auto; }
.s-opts { display: grid; gap: 8px; margin: 12px 0 0; padding: 0; list-style: none; }
@media (min-width: 640px) { .s-opts { grid-template-columns: repeat(2, 1fr); } }
.s-opts li { padding: 8px 12px; border-radius: .8rem; border: 1px solid var(--line); background: #fff; }
.s-lock { margin-top: 16px; padding: 24px 20px; border-radius: 1.25rem; text-align: center; background: linear-gradient(160deg, #2A2670, #1B1A3A); color: #fff; }
.s-lock p { color: rgba(255,255,255,.82); }
.s-lock .s-btn { margin-top: 16px; }

/* Danh sách liên kết */
.s-list { list-style: none; padding: 0; margin: 12px 0 0; display: grid; gap: 8px; }
.s-list a { display: flex; justify-content: space-between; gap: 12px; align-items: center; min-height: 48px; padding: 10px 14px; border-radius: .9rem; background: #fff; border: 1px solid var(--line); text-decoration: none; color: var(--ink); font-weight: 600; }
.s-list a:hover { border-color: #C5CAFF; }
.s-list small { color: var(--ink-soft); font-weight: 500; font-size: .85rem; white-space: nowrap; }
.s-pillar { border-left: 6px solid var(--brand); }

/* Bài viết */
.s-article { background: #fff; border: 1px solid var(--line); border-radius: 1.5rem; padding: 24px 20px; margin-top: -24px; position: relative; box-shadow: var(--shadow); }
@media (min-width: 768px) { .s-article { padding: 40px 48px; } }
.s-article h2 { margin-top: 32px; }
.s-article h3 { margin-top: 20px; }
.s-article ul, .s-article ol { padding-left: 1.3rem; }
.s-article li { margin-top: .35rem; }
.s-article blockquote { margin: 20px 0; padding: 14px 18px; border-left: 5px solid var(--accent); background: var(--accent-50); border-radius: 0 1rem 1rem 0; font-weight: 600; }
.s-meta { font-size: .9rem; color: var(--ink-soft); }

/* Hỏi đáp */
.s-faq { display: grid; gap: 10px; margin-top: 16px; }
.s-faq details { background: #fff; border: 1px solid var(--line); border-radius: 1rem; padding: 0 16px; }
.s-faq summary { cursor: pointer; min-height: 52px; display: flex; align-items: center; font-weight: 700; list-style: none; padding: 12px 24px 12px 0; position: relative; }
.s-faq summary::-webkit-details-marker { display: none; }
.s-faq summary::after { content: "+"; position: absolute; right: 0; font-size: 1.4rem; color: var(--brand); }
.s-faq details[open] summary::after { content: "−"; }
.s-faq details > div { padding: 0 0 16px; color: var(--ink-soft); }

/* CTA cuối trang */
.s-cta { margin-top: 56px; border-radius: 1.5rem; padding: 32px 20px; text-align: center; color: #fff; background: radial-gradient(120% 120% at 80% 0%, #3B36A8 0%, #2A2670 50%, #1B1A3A 100%); }
.s-cta h2 { color: #fff; }
.s-cta p { color: rgba(255,255,255,.85); }
.s-cta .s-btn { margin-top: 18px; }
.s-price { font-size: 1.6rem; font-weight: 700; }
.s-price s { font-size: 1rem; opacity: .65; margin-left: 6px; font-weight: 500; }

/* Footer */
.s-footer { margin-top: 64px; padding: 32px 0 40px; border-top: 1px solid var(--line); background: #fff; font-size: .92rem; }
.s-footer .s-grid { gap: 24px; }
.s-footer h3 { font-size: .95rem; margin-bottom: 8px; }
.s-footer ul { list-style: none; padding: 0; margin: 0; display: grid; gap: 4px; }
.s-footer a { color: var(--ink-soft); text-decoration: none; display: inline-flex; min-height: 32px; align-items: center; }
.s-footer a:hover { color: var(--brand); }

@media (prefers-reduced-motion: reduce) { .s-btn, a.s-card { transition: none; } }

/* Khẩu hiệu ngắn (hero tối) */
.s-slogan { display: flex; flex-wrap: wrap; gap: 2px 14px; margin: 18px 0 4px; font-size: .95rem; color: rgba(255,255,255,.82); }
.s-slogan b { color: #FFAE72; font-size: 1.15em; font-weight: 800; }

/* Thẻ kỳ thi Toán: viền chàm */
.s-card.s-math { border-top: 4px solid var(--brand); }

/* Bảng giá */
.s-plans { display: grid; gap: 16px; margin-top: 20px; }
@media (min-width: 640px) { .s-plans { grid-template-columns: repeat(2, 1fr); } }
@media (min-width: 1024px) { .s-plans { grid-template-columns: repeat(4, 1fr); } }
.s-plan { position: relative; display: flex; flex-direction: column; background: #fff; border: 1px solid var(--line); border-radius: 1.25rem; box-shadow: var(--shadow); padding: 22px 18px; }
.s-plan h3 { font-size: 1.1rem; }
.s-plan .s-price { margin-top: 8px; color: var(--ink); }
.s-plan .s-price small { font-size: .85rem; font-weight: 500; color: var(--ink-soft); }
.s-plan ul { list-style: none; padding: 0; margin: 14px 0 18px; display: grid; gap: 6px; font-size: .95rem; }
.s-plan li::before { content: "✓ "; color: var(--mint); font-weight: 700; }
.s-plan .s-btn { margin-top: auto; justify-content: center; }
.s-plan-vip { border: 2px solid var(--accent); background: linear-gradient(180deg, var(--accent-50), #fff 45%); }
.s-plan-tag { position: absolute; top: -12px; left: 18px; background: var(--accent); color: #fff; font-size: .8rem; font-weight: 700; padding: .2rem .7rem; border-radius: 999px; }

/* Từ khoá */
.s-terms { display: grid; gap: 10px; margin-top: 14px; grid-template-columns: repeat(auto-fill, minmax(220px, 1fr)); }
.s-term { display: flex; gap: 10px; align-items: center; min-height: 56px; padding: 10px 14px; border-radius: 1rem; background: #fff; border: 1px solid var(--line); text-decoration: none; color: var(--ink); }
.s-term:hover { border-color: #C5CAFF; }
.s-term b { display: block; font-size: 1rem; }
.s-term small { display: block; color: var(--ink-soft); font-size: .85rem; }
.s-term .s-emo { font-size: 1.5rem; }
.s-ipa { font-family: ui-monospace, 'SF Mono', Menlo, monospace; color: var(--ink-soft); }
.s-example { border-left: 4px solid var(--brand-100); padding: 8px 14px; margin: 10px 0 0; background: #fff; border-radius: 0 .75rem .75rem 0; font-style: italic; }
.s-example mark { background: var(--accent-50); color: #C85412; font-style: normal; font-weight: 700; padding: 0 2px; border-radius: 4px; }
.s-letters { display: flex; flex-wrap: wrap; gap: 6px; margin-top: 12px; }
.s-letters a { display: inline-flex; align-items: center; justify-content: center; min-width: 44px; min-height: 44px; border-radius: .75rem; background: #fff; border: 1px solid var(--line); text-decoration: none; font-weight: 700; }

/* Cam kết "100% đề thi do LuyệnToán.net tự biên soạn" (pledgeBlock() trong server/seo.js) */
.s-pledge { background: linear-gradient(160deg, #F1F0FF 0%, #FFFFFF 55%, #FFF4EC 100%); border: 1px solid #DCD9FF; border-radius: 1.5rem; padding: 24px 20px; }
@media (min-width: 640px) { .s-pledge { padding: 32px; } }
.s-pledge h2 { margin-top: 6px; }
.s-pledge-ip { margin-top: 16px; padding: 14px 16px; border-radius: 1rem; background: #FFF4EC; border: 1px dashed #FFC59E; color: var(--ink); }