/* ===== KHTN Vật Lý – Shared Styles ===== */
@import url('https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&family=Space+Grotesk:wght@500;600;700&display=swap');

:root {
  --bg: #f5f7fa; --bg2: #ffffff; --fg: #1a1a2e; --fg2: #4a4a6a;
  --ac: #2563eb; --ac2: #7c3aed; --ac3: #059669;
  --ok: #16a34a; --bad: #dc2626; --warn: #d97706;
  --line: #e2e8f0; --card: #ffffff; --card-shadow: rgba(0,0,0,.06);
  --hint-bg: #fef3c7; --hint-fg: #92400e;
  --grad1: #2563eb; --grad2: #7c3aed;
  --radius: 14px;
}
@media(prefers-color-scheme:dark){:root:not([data-theme="light"]){
  --bg:#0f172a;--bg2:#1e293b;--fg:#e2e8f0;--fg2:#94a3b8;
  --ac:#60a5fa;--ac2:#a78bfa;--ac3:#34d399;
  --ok:#4ade80;--bad:#f87171;--warn:#fbbf24;
  --line:#334155;--card:#1e293b;--card-shadow:rgba(0,0,0,.3);
  --hint-bg:#78350f;--hint-fg:#fef3c7;
  --grad1:#3b82f6;--grad2:#8b5cf6;
  color-scheme:dark;
}}
:root[data-theme="dark"]{
  --bg:#0f172a;--bg2:#1e293b;--fg:#e2e8f0;--fg2:#94a3b8;
  --ac:#60a5fa;--ac2:#a78bfa;--ac3:#34d399;
  --ok:#4ade80;--bad:#f87171;--warn:#fbbf24;
  --line:#334155;--card:#1e293b;--card-shadow:rgba(0,0,0,.3);
  --hint-bg:#78350f;--hint-fg:#fef3c7;
  --grad1:#3b82f6;--grad2:#8b5cf6;
  color-scheme:dark;
}

*,*::before,*::after{box-sizing:border-box}
body{margin:0;font-family:'Inter',system-ui,sans-serif;font-size:1rem;line-height:1.6;
  background:var(--bg);color:var(--fg);-webkit-font-smoothing:antialiased}
h1,h2,h3,h4{font-family:'Space Grotesk','Inter',sans-serif;line-height:1.3;text-wrap:balance}
img{max-width:100%}
a{color:var(--ac);text-decoration:none}
a:hover{text-decoration:underline}

/* ===== Layout ===== */
.container{max-width:960px;margin:0 auto;padding:0 16px}

/* ===== Home page hero ===== */
.hero{text-align:center;padding:48px 16px 36px;background:linear-gradient(135deg,var(--grad1),var(--grad2));color:#fff;position:relative;overflow:hidden}
.hero::after{content:'';position:absolute;inset:0;background:radial-gradient(circle at 30% 70%,rgba(255,255,255,.12),transparent 60%)}
.hero h1{font-size:clamp(1.6rem,4vw,2.4rem);margin:0 0 8px;position:relative;z-index:1}
.hero p{font-size:1.05rem;opacity:.9;margin:0;position:relative;z-index:1}

/* ===== Grade cards on home ===== */
.grades{display:grid;grid-template-columns:repeat(auto-fit,minmax(200px,1fr));gap:16px;padding:32px 16px 48px}
.grade-card{background:var(--card);border:1px solid var(--line);border-radius:var(--radius);
  padding:24px 20px;text-align:center;text-decoration:none;color:var(--fg);
  box-shadow:0 2px 8px var(--card-shadow);transition:transform .2s,box-shadow .2s}
.grade-card:hover{transform:translateY(-4px);box-shadow:0 8px 24px var(--card-shadow);text-decoration:none}
.grade-card .icon{font-size:2.4rem;margin-bottom:8px}
.grade-card h2{margin:0 0 6px;font-size:1.25rem}
.grade-card p{margin:0;font-size:.9rem;color:var(--fg2)}

/* ===== Grade page header ===== */
.grade-header{padding:32px 16px 24px;background:linear-gradient(135deg,var(--grad1),var(--grad2));color:#fff}
.grade-header h1{margin:0 0 4px;font-size:clamp(1.4rem,3.5vw,2rem)}
.grade-header p{margin:0;opacity:.9;font-size:.95rem}
.grade-header .back{color:rgba(255,255,255,.85);font-size:.9rem;display:inline-block;margin-bottom:8px}
.grade-header .back:hover{color:#fff}

/* ===== Lesson navigation ===== */
.chapter{margin:24px 0 8px;padding:0 4px}
.chapter h3{font-size:1rem;color:var(--ac);margin:0;font-weight:600;letter-spacing:.02em;text-transform:uppercase}
.lessons{display:grid;grid-template-columns:repeat(auto-fill,minmax(280px,1fr));gap:12px;padding:8px 0}
.lesson-card{display:flex;align-items:center;gap:12px;background:var(--card);border:1px solid var(--line);
  border-radius:var(--radius);padding:14px 16px;cursor:pointer;transition:border-color .2s,box-shadow .2s;text-decoration:none;color:var(--fg)}
.lesson-card:hover{border-color:var(--ac);box-shadow:0 2px 12px var(--card-shadow);text-decoration:none}
.lesson-card .num{font-family:'Space Grotesk',sans-serif;font-size:1.3rem;font-weight:700;color:var(--ac);min-width:36px;text-align:center}
.lesson-card .title{font-size:.95rem;font-weight:500}

/* ===== Lesson detail ===== */
.lesson-detail{display:none;animation:fadeIn .3s}
.lesson-detail.active{display:block}
@keyframes fadeIn{from{opacity:0;transform:translateY(10px)}to{opacity:1;transform:none}}
.lesson-back{display:inline-flex;align-items:center;gap:4px;color:var(--ac);font-size:.9rem;margin:16px 0 8px;cursor:pointer;background:none;border:none;font:inherit}
.lesson-back:hover{text-decoration:underline}

/* ===== Content boxes ===== */
.box{background:var(--card);border:1px solid var(--line);border-radius:var(--radius);padding:20px;margin:12px 0;
  box-shadow:0 1px 4px var(--card-shadow)}
.box h3{margin:0 0 10px;font-size:1.05rem;color:var(--ac)}
.box ul{margin:0;padding-left:20px}
.box ul li{margin:4px 0;line-height:1.55}
.box ol{margin:0;padding-left:20px}
.box ol li{margin:4px 0;line-height:1.55}
.formula{background:var(--bg);border-left:4px solid var(--ac);padding:10px 16px;border-radius:0 8px 8px 0;
  font-family:'Space Grotesk',monospace;font-size:1.05rem;margin:10px 0;overflow-x:auto}

/* ===== Experiment area ===== */
.exp-area{position:relative}
canvas{display:block;max-width:100%;height:auto;background:var(--bg2);border:1px solid var(--line);border-radius:10px;margin:8px 0}
.data-row{display:flex;flex-wrap:wrap;gap:10px;margin:10px 0}
.data-item{background:var(--bg);border-radius:8px;padding:8px 14px;font-size:.9rem;flex:1;min-width:140px}
.data-item label{display:block;font-size:.8rem;color:var(--fg2);margin-bottom:2px}
.data-item b{font-family:'Space Grotesk',sans-serif;font-size:1.1rem;color:var(--fg)}
.data-input{display:flex;flex-wrap:wrap;gap:8px;align-items:center;margin:8px 0}
.data-input label{font-size:.85rem;color:var(--fg2)}
.data-input input[type="number"]{width:100px;padding:8px 10px;border:1.5px solid var(--line);border-radius:8px;
  background:var(--bg);color:var(--fg);font:inherit;font-size:1rem}
.data-input input[type="number"]:focus{outline:none;border-color:var(--ac)}
.data-input span{font-size:.9rem;color:var(--fg2)}

/* ===== Buttons ===== */
.btn{display:inline-flex;align-items:center;gap:6px;padding:10px 20px;border:none;border-radius:10px;
  font:inherit;font-size:.95rem;font-weight:600;cursor:pointer;transition:opacity .2s;color:#fff;background:var(--ac)}
.btn:hover{opacity:.88}
.btn.s{background:var(--ac3)}
.btn.warn{background:var(--warn)}
.btn.danger{background:var(--bad)}
.btn:disabled{opacity:.5;cursor:not-allowed}

/* ===== Exercise section ===== */
.exercise{background:var(--card);border:1px solid var(--line);border-radius:var(--radius);padding:20px;margin:12px 0}
.exercise h3{margin:0 0 12px;font-size:1.05rem;color:var(--ac2)}
.exercise .q{margin:8px 0 12px;font-weight:500}
.exercise .choices{display:flex;flex-direction:column;gap:8px;margin:8px 0}
.exercise .choice{display:flex;align-items:center;gap:10px;padding:10px 14px;background:var(--bg);
  border:1.5px solid var(--line);border-radius:10px;cursor:pointer;transition:border-color .2s}
.exercise .choice:hover{border-color:var(--ac)}
.exercise .choice.selected{border-color:var(--ac);background:color-mix(in srgb,var(--ac) 8%,var(--bg))}
.exercise .choice.correct{border-color:var(--ok);background:color-mix(in srgb,var(--ok) 10%,var(--bg))}
.exercise .choice.wrong{border-color:var(--bad);background:color-mix(in srgb,var(--bad) 8%,var(--bg))}
.exercise .choice input[type="radio"]{accent-color:var(--ac);width:18px;height:18px}

.ask-row{display:flex;flex-wrap:wrap;align-items:center;gap:8px;margin:10px 0}
.ask-row input[type="number"]{width:120px;padding:8px 10px;border:1.5px solid var(--line);border-radius:8px;
  background:var(--bg);color:var(--fg);font:inherit;font-size:1rem}
.ask-row input:focus{outline:none;border-color:var(--ac)}
.ask-row span{font-size:.9rem;color:var(--fg2)}

.hint-box{background:var(--hint-bg);color:var(--hint-fg);padding:10px 14px;border-radius:8px;margin:8px 0;
  font-size:.92rem;display:none}
.hint-box.show{display:block;animation:fadeIn .3s}

.result-msg{padding:8px 14px;border-radius:8px;margin:8px 0;font-weight:500;font-size:.95rem}
.result-msg.ok{background:color-mix(in srgb,var(--ok) 12%,var(--bg));color:var(--ok)}
.result-msg.no{background:color-mix(in srgb,var(--bad) 10%,var(--bg));color:var(--bad)}

/* ===== Tabs within lesson ===== */
.tabs{display:flex;gap:0;border-bottom:2px solid var(--line);margin:16px 0 12px}
.tab{padding:10px 18px;font-size:.92rem;font-weight:600;cursor:pointer;border-bottom:3px solid transparent;
  color:var(--fg2);transition:color .2s,border-color .2s;background:none;border-top:none;border-left:none;border-right:none;font:inherit;font-weight:600}
.tab:hover{color:var(--fg)}
.tab.active{color:var(--ac);border-bottom-color:var(--ac)}
.tab-content{display:none}
.tab-content.active{display:block;animation:fadeIn .3s}

/* ===== Footer ===== */
.site-footer{text-align:center;padding:24px 16px;font-size:.85rem;color:var(--fg2);border-top:1px solid var(--line);margin-top:32px}

/* ===== Responsive ===== */
@media(max-width:500px){
  .hero{padding:32px 16px 28px}
  .grades{grid-template-columns:1fr 1fr;gap:10px;padding:20px 12px 32px}
  .grade-card{padding:16px 12px}
  .lessons{grid-template-columns:1fr}
  .data-row{flex-direction:column}
  .tabs{overflow-x:auto;-webkit-overflow-scrolling:touch}
  .tab{white-space:nowrap;flex-shrink:0}
}