/* Mathvera — giao diện chung cho trang chủ (giáo viên) và trang hướng dẫn. */
:root{
  --dark:#121220; --dark2:#171728; --card:#1b1b30; --card2:#212139; --line:rgba(255,255,255,.08);
  --coral:#E15C3A; --teal:#4ECDC4; --gold:#FFD700; --cream:#FFF5F0;
  --txt:#EDEAF2; --mut:#a6a3b8; --radius:18px; --max:1160px;
}
*{box-sizing:border-box;margin:0;padding:0}
html{scroll-behavior:smooth;scroll-padding-top:84px}
body{font-family:'Be Vietnam Pro',system-ui,sans-serif;background:var(--dark);color:var(--txt);line-height:1.65;-webkit-font-smoothing:antialiased;overflow-x:hidden}
a{color:inherit;text-decoration:none}
img,video{max-width:100%;display:block}
.wrap{max-width:var(--max);margin:0 auto;padding:0 22px}
h1,h2,h3,h4{font-weight:800;line-height:1.15;letter-spacing:-.02em;color:var(--cream)}
.grad{background:linear-gradient(100deg,var(--coral),var(--gold) 60%,var(--teal));-webkit-background-clip:text;background-clip:text;-webkit-text-fill-color:transparent}
.eyebrow{display:inline-block;font-size:13px;font-weight:700;letter-spacing:.14em;text-transform:uppercase;color:var(--teal);background:rgba(78,205,196,.1);border:1px solid rgba(78,205,196,.25);padding:6px 14px;border-radius:999px;margin-bottom:18px}
.btn{display:inline-flex;align-items:center;gap:8px;font-weight:700;font-size:16px;padding:14px 26px;border-radius:999px;cursor:pointer;border:1px solid transparent;transition:transform .2s ease,box-shadow .2s ease,background .2s ease}
.btn-primary{background:linear-gradient(100deg,var(--coral),#f0793c);color:#fff;box-shadow:0 10px 30px rgba(225,92,58,.35)}
.btn-primary:hover{transform:translateY(-2px);box-shadow:0 16px 40px rgba(225,92,58,.5)}
.btn-ghost{background:rgba(255,255,255,.04);color:var(--cream);border-color:var(--line)}
.btn-ghost:hover{background:rgba(255,255,255,.09);transform:translateY(-2px)}

/* header */
header{position:sticky;top:0;z-index:50;backdrop-filter:blur(14px);background:rgba(18,18,32,.78);border-bottom:1px solid var(--line)}
nav{display:flex;align-items:center;justify-content:space-between;height:68px}
.logo{display:flex;align-items:center;gap:10px;font-weight:800;font-size:20px;color:var(--cream)}
.logo img{width:34px;height:34px;border-radius:8px}
.navlinks{display:flex;gap:26px;align-items:center}
.navlinks a{color:var(--mut);font-size:15px;font-weight:500;transition:color .2s}
.navlinks a:hover,.navlinks a.on{color:var(--cream)}
.navlinks .btn-primary{color:#fff;padding:10px 20px;font-size:15px}
.lang{display:flex;border:1px solid var(--line);border-radius:999px;overflow:hidden}
.lang a{padding:4px 11px;font-size:12.5px;font-weight:700}
.lang a.on{background:rgba(255,255,255,.1);color:var(--cream)}
.menu-btn{display:none;background:none;border:none;color:var(--cream);font-size:26px;cursor:pointer}

/* sections */
.sec{padding:96px 0}
.sec.alt{background:var(--dark2)}
.sec-head{text-align:center;max-width:780px;margin:0 auto 52px}
.sec-head h2{font-size:clamp(28px,4vw,42px);margin-bottom:14px}
.sec-head p{color:var(--mut);font-size:17.5px}

/* hero */
.hero{padding:76px 0 40px;text-align:center;background:radial-gradient(900px 420px at 50% -10%,rgba(78,205,196,.13),transparent 70%),radial-gradient(700px 360px at 85% 20%,rgba(225,92,58,.12),transparent 70%)}
.hero h1{font-size:clamp(34px,5.6vw,62px);max-width:900px;margin:0 auto 20px}
.hero .sub{font-size:clamp(16.5px,2vw,19.5px);color:var(--mut);max-width:760px;margin:0 auto 30px}
.hero-cta{display:flex;gap:14px;justify-content:center;flex-wrap:wrap}
.hero .note{margin-top:16px;font-size:14px;color:var(--mut)}

/* khung video kiểu cửa sổ */
.frame{border-radius:16px;border:1px solid var(--line);background:#f7f7f9;box-shadow:0 30px 90px rgba(0,0,0,.5);overflow:hidden}
.frame .bar{display:flex;gap:7px;padding:10px 14px;background:var(--dark2);border-bottom:1px solid var(--line)}
.frame .bar i{width:11px;height:11px;border-radius:50%;display:block}
.frame video,.frame img{width:100%;height:auto}
.hero .frame{max-width:1000px;margin:48px auto 0}
.cap{margin:12px 4px 0;font-size:14px;color:var(--mut);text-align:center}

/* dải số liệu */
.stats{border-top:1px solid var(--line);border-bottom:1px solid var(--line);background:var(--dark2)}
.stats .wrap{display:grid;grid-template-columns:repeat(4,1fr)}
.stat{padding:26px 18px;text-align:center;border-right:1px solid var(--line)}
.stat:last-child{border-right:none}
.stat b{display:block;font-size:17px;color:var(--cream)}
.stat span{font-size:14px;color:var(--mut)}

/* hàng khoe tính năng (chữ | video), so le trái phải */
.show{display:grid;grid-template-columns:1fr 1.35fr;gap:48px;align-items:center;margin-bottom:84px}
.show:last-child{margin-bottom:0}
.show.flip .txt{order:2}
.show .txt h3{font-size:clamp(23px,2.8vw,30px);margin-bottom:12px}
.show .txt p{color:var(--mut);font-size:16.5px;margin-bottom:14px}
.show .kicker{font-size:13px;font-weight:700;letter-spacing:.12em;text-transform:uppercase;color:var(--coral);margin-bottom:10px;display:block}
.ticks{list-style:none;display:grid;gap:8px}
.ticks li{padding-left:26px;position:relative;color:var(--txt);font-size:15.5px}
.ticks li:before{content:"✓";position:absolute;left:0;top:0;color:var(--teal);font-weight:800}

/* lưới thẻ */
.cards{display:grid;grid-template-columns:repeat(3,1fr);gap:18px}
.card{background:var(--card);border:1px solid var(--line);border-radius:var(--radius);padding:26px}
.card .ic{font-size:26px;margin-bottom:10px}
.card h3{font-size:18.5px;margin-bottom:8px}
.card p{color:var(--mut);font-size:15px}
.card a.more{display:inline-block;margin-top:12px;color:var(--teal);font-weight:700;font-size:14.5px}

/* hai cách dạy */
.ways{display:grid;grid-template-columns:1fr 1fr;gap:20px}
.way{background:var(--card);border:1px solid var(--line);border-radius:var(--radius);padding:30px}
.way .n{display:inline-flex;width:34px;height:34px;border-radius:50%;align-items:center;justify-content:center;background:rgba(78,205,196,.14);color:var(--teal);font-weight:800;margin-bottom:12px}
.way h3{font-size:21px;margin-bottom:10px}
.way ol{padding-left:20px;color:var(--txt);display:grid;gap:7px;font-size:15.5px}
.way p{color:var(--mut);margin-top:12px;font-size:15px}

/* CTA hướng dẫn */
.guidecta{background:linear-gradient(135deg,rgba(78,205,196,.12),rgba(225,92,58,.1));border:1px solid var(--line);border-radius:24px;padding:44px;display:grid;grid-template-columns:1.1fr 1fr;gap:36px;align-items:center}
.guidecta h2{font-size:clamp(26px,3.4vw,36px);margin-bottom:12px}
.guidecta p{color:var(--mut);margin-bottom:22px}
.toclist{list-style:none;columns:2;gap:20px}
.toclist li{break-inside:avoid;padding:6px 0;border-bottom:1px solid var(--line);font-size:15px}
.toclist a:hover{color:var(--teal)}

/* hỏi đáp */
.faq{max-width:800px;margin:0 auto;display:grid;gap:12px}
.q{background:var(--card);border:1px solid var(--line);border-radius:14px}
.q summary{cursor:pointer;padding:19px 24px;font-weight:700;font-size:16.5px;color:var(--cream);list-style:none;display:flex;justify-content:space-between;align-items:center;gap:12px}
.q summary::-webkit-details-marker{display:none}
.q summary:after{content:"+";font-size:24px;color:var(--coral);transition:transform .2s}
.q[open] summary:after{transform:rotate(45deg)}
.q .a{padding:0 24px 20px;color:var(--mut);font-size:15.5px}

.finalcta{padding:96px 0;text-align:center;background:radial-gradient(700px 300px at 50% 100%,rgba(225,92,58,.16),transparent 70%)}
.finalcta h2{font-size:clamp(28px,4vw,44px);margin-bottom:14px}
.finalcta p{color:var(--mut);font-size:17.5px;margin-bottom:28px}

footer{border-top:1px solid var(--line);background:var(--dark2);padding:44px 0 30px}
.foot{display:flex;justify-content:space-between;flex-wrap:wrap;gap:24px}
.foot p{color:var(--mut);font-size:14px;margin-top:10px;max-width:340px}
.foot-links{display:flex;gap:24px;flex-wrap:wrap}
.foot-links a{color:var(--mut);font-size:14px}
.foot-links a:hover{color:var(--cream)}
.copy{margin-top:26px;font-size:13px;color:var(--mut)}

.reveal{opacity:0;transform:translateY(24px);transition:opacity .6s ease,transform .6s ease}
.reveal.in{opacity:1;transform:none}

/* ===== trang hướng dẫn ===== */
.doc{display:grid;grid-template-columns:260px minmax(0,1fr);gap:48px;padding-top:40px;padding-bottom:80px}
.toc{position:sticky;top:88px;align-self:start;max-height:calc(100vh - 110px);overflow:auto;padding-right:6px}
.toc b{display:block;font-size:12.5px;letter-spacing:.12em;text-transform:uppercase;color:var(--mut);margin:0 0 10px}
.toc ol{list-style:none;display:grid;gap:2px}
.toc a{display:block;padding:6px 10px;border-radius:8px;color:var(--mut);font-size:14.5px;border-left:2px solid transparent}
.toc a:hover{color:var(--cream);background:rgba(255,255,255,.04)}
.toc a.on{color:var(--cream);border-left-color:var(--teal);background:rgba(78,205,196,.07)}
.toc .sub a{padding-left:22px;font-size:13.5px}
.doc-main{min-width:0}
.doc-intro h1{font-size:clamp(30px,4.4vw,46px);margin-bottom:14px}
.doc-intro p{color:var(--mut);font-size:17.5px;max-width:760px}
.chap{padding-top:56px}
.chap>h2{font-size:clamp(25px,3.2vw,33px);margin-bottom:8px;display:flex;gap:12px;align-items:baseline}
.chap>h2 .no{color:var(--teal);font-size:.7em}
.chap>.lead{color:var(--mut);font-size:16.5px;margin-bottom:22px;max-width:780px}
.chap h3{font-size:20px;margin:30px 0 10px}
.chap h4{font-size:16.5px;margin:20px 0 8px;color:var(--gold)}
.chap p,.chap li{font-size:15.8px}
.chap p{margin-bottom:12px}
.chap ol,.chap ul{padding-left:22px;display:grid;gap:6px;margin-bottom:14px}
.chap .frame{margin:18px 0 8px}
.chap .cap{text-align:left;margin:6px 2px 18px}
.kbd,kbd{display:inline-block;font-family:ui-monospace,Menlo,monospace;font-size:13px;padding:1px 7px;border-radius:6px;background:var(--card2);border:1px solid rgba(255,255,255,.14);border-bottom-width:2px;color:var(--cream);white-space:nowrap}
.ui{font-weight:700;color:var(--cream);background:rgba(255,255,255,.06);border:1px solid var(--line);border-radius:6px;padding:0 6px;white-space:nowrap}
.tip{background:rgba(78,205,196,.08);border:1px solid rgba(78,205,196,.25);border-radius:12px;padding:14px 18px;margin:14px 0;font-size:15.3px}
.tip b{color:var(--teal)}
.warn{background:rgba(225,92,58,.08);border:1px solid rgba(225,92,58,.3);border-radius:12px;padding:14px 18px;margin:14px 0;font-size:15.3px}
.warn b{color:var(--coral)}
.tbl{width:100%;border-collapse:collapse;margin:12px 0 18px;font-size:15px;background:var(--card);border-radius:12px;overflow:hidden}
.tbl th,.tbl td{padding:10px 14px;border-bottom:1px solid var(--line);text-align:left;vertical-align:top}
.tbl th{background:var(--card2);color:var(--cream);font-size:13.5px;letter-spacing:.04em}
.tbl tr:last-child td{border-bottom:none}
.tbl td:first-child{font-weight:700;color:var(--cream);white-space:nowrap}
.tblwrap{overflow-x:auto}
.two{display:grid;grid-template-columns:1fr 1fr;gap:18px}
.shot{border-radius:12px;border:1px solid var(--line);overflow:hidden;background:#f7f7f9;margin:14px 0 6px}
.shot img{width:100%}
.shot.sm{max-width:560px}
.two.side{grid-template-columns:230px minmax(0,1fr);align-items:start}

@media(max-width:980px){
  .show,.show.flip{grid-template-columns:1fr;gap:22px}
  .show.flip .txt{order:0}
  .cards{grid-template-columns:1fr 1fr}
  .guidecta{grid-template-columns:1fr;padding:30px}
  .doc{grid-template-columns:minmax(0,1fr)}
  .toc{position:static;max-height:none;border:1px solid var(--line);border-radius:14px;padding:16px;background:var(--card)}
}
@media(max-width:760px){
  .menu-btn{display:block}
  .navlinks{display:none;position:absolute;top:100%;left:0;right:0;flex-direction:column;align-items:stretch;gap:0;background:rgba(18,18,32,.98);border-bottom:1px solid var(--line);padding:8px 22px 18px}
  .navlinks.open{display:flex}
  .navlinks a{padding:12px 4px;font-size:16px;border-bottom:1px solid var(--line)}
  .navlinks .lang{align-self:flex-start;margin:12px 0 4px;border-bottom:none}
  .navlinks .lang a{border-bottom:none;padding:5px 14px}
  .navlinks .btn-primary{margin-top:12px;justify-content:center;border-bottom:none}
  .stats .wrap{grid-template-columns:1fr 1fr}
  .stat:nth-child(2){border-right:none}
  .stat:nth-child(-n+2){border-bottom:1px solid var(--line)}
  .cards,.ways,.two,.two.side{grid-template-columns:minmax(0,1fr)}
  .two.side .shot{max-width:260px}
  .tbl td:first-child{white-space:normal}
  .toclist{columns:1}
  .sec{padding:68px 0}
}