/* ── Reset & Variables ── */
*{box-sizing:border-box;margin:0;padding:0}
:root{
  --bg:#f7f8fc;--white:#fff;--dark:#1c1d1f;--dark2:#2d2f31;
  --gray1:#6a6f73;--gray2:#9da3a7;--gray3:#d1d7dc;--gray4:#e8eaed;--gray5:#f4f5f7;
  --primary:#306ee8;--primary-dark:#2255c4;--primary-light:#e8effd;
  --accent:#00b894;--accent-dark:#009d80;--accent-light:#e6f9f4;
  --warning:#fdcb6e;--danger:#e17055;--purple:#6c5ce7;
  --shadow:0 2px 8px rgba(0,0,0,.08);--shadow-lg:0 4px 20px rgba(0,0,0,.12);
  --radius:8px;--radius-lg:12px;
  --nav-h:64px;--sidebar-w:320px;
  --font:'Inter',system-ui,-apple-system,sans-serif;
}
html,body{height:100%;overflow:hidden}
body{font-family:var(--font);background:var(--bg);color:var(--dark);font-size:14px;line-height:1.6}
a{color:var(--primary);text-decoration:none}
a:hover{text-decoration:underline}
button{cursor:pointer;font-family:var(--font)}
img{max-width:100%}

/* ── Scrollbar ── */
::-webkit-scrollbar{width:6px}
::-webkit-scrollbar-track{background:transparent}
::-webkit-scrollbar-thumb{background:var(--gray3);border-radius:3px}
::-webkit-scrollbar-thumb:hover{background:var(--gray2)}

/* ── Navbar ── */
.navbar{position:fixed;top:0;left:0;right:0;height:var(--nav-h);background:var(--dark);display:flex;align-items:center;padding:0 24px;z-index:100;gap:24px}
.nav-logo{font-size:18px;font-weight:800;color:var(--white);letter-spacing:-.3px;white-space:nowrap}
.nav-logo span{color:var(--accent)}
.nav-links{display:flex;gap:4px;margin-left:16px}
.nav-link{color:var(--gray2);font-size:13px;font-weight:500;padding:8px 14px;border-radius:6px;transition:all .15s;border:none;background:none}
.nav-link:hover,.nav-link.active{color:var(--white);background:rgba(255,255,255,.1);text-decoration:none}
.nav-progress{margin-left:auto;display:flex;align-items:center;gap:10px;color:var(--gray2);font-size:12px;font-weight:500}
.nav-progress-bar{width:120px;height:6px;background:rgba(255,255,255,.15);border-radius:3px;overflow:hidden}
.nav-progress-fill{height:100%;background:var(--accent);border-radius:3px;transition:width .4s ease}

/* ── Main Layout ── */
.main{margin-top:var(--nav-h);height:calc(100vh - var(--nav-h));overflow-y:auto;padding:0}

/* ── Dashboard ── */
.dashboard{max-width:1200px;margin:0 auto;padding:32px 24px 64px}
.hero{background:linear-gradient(135deg,#1c1d1f 0%,#2d2f31 100%);border-radius:var(--radius-lg);padding:48px;margin-bottom:40px;color:var(--white);position:relative;overflow:hidden}
.hero::after{content:'';position:absolute;top:-50%;right:-10%;width:400px;height:400px;background:radial-gradient(circle,rgba(0,184,148,.15),transparent 70%);pointer-events:none}
.hero h1{font-size:32px;font-weight:800;margin-bottom:8px;letter-spacing:-.5px}
.hero p{font-size:15px;color:var(--gray2);max-width:600px;line-height:1.7}
.hero-stats{display:flex;gap:32px;margin-top:24px}
.hero-stat{text-align:center}
.hero-stat .val{font-size:24px;font-weight:700;color:var(--accent)}
.hero-stat .lbl{font-size:11px;color:var(--gray2);text-transform:uppercase;letter-spacing:1px;margin-top:2px}

.section-title{font-size:20px;font-weight:700;margin-bottom:20px;color:var(--dark)}

/* ── Course Cards ── */
.course-grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(320px,1fr));gap:20px}
.course-card{background:var(--white);border-radius:var(--radius-lg);overflow:hidden;box-shadow:var(--shadow);transition:all .2s;cursor:pointer;border:2px solid transparent}
.course-card:hover{transform:translateY(-4px);box-shadow:var(--shadow-lg);border-color:var(--primary)}
.card-banner{height:140px;display:flex;align-items:center;justify-content:center;font-size:48px;position:relative}
.card-banner .card-level{position:absolute;top:12px;right:12px;font-size:11px;font-weight:600;padding:4px 10px;border-radius:20px;background:rgba(255,255,255,.9);color:var(--dark2)}
.card-body{padding:20px}
.card-body h3{font-size:16px;font-weight:700;margin-bottom:6px;line-height:1.3}
.card-body p{font-size:13px;color:var(--gray1);margin-bottom:16px;line-height:1.5;display:-webkit-box;-webkit-line-clamp:2;-webkit-box-orient:vertical;overflow:hidden}
.card-meta{display:flex;align-items:center;gap:16px;font-size:12px;color:var(--gray2);margin-bottom:12px}
.card-progress{height:4px;background:var(--gray4);border-radius:2px;overflow:hidden}
.card-progress-fill{height:100%;border-radius:2px;transition:width .4s ease}
.card-progress-label{font-size:11px;color:var(--gray1);margin-top:6px;font-weight:500}

/* ── Course Detail ── */
.course-detail{max-width:900px;margin:0 auto;padding:32px 24px 64px}
.back-btn{display:inline-flex;align-items:center;gap:6px;font-size:13px;color:var(--gray1);font-weight:500;margin-bottom:20px;padding:6px 12px;border-radius:6px;background:none;border:1px solid var(--gray3);transition:all .15s}
.back-btn:hover{color:var(--dark);border-color:var(--dark);text-decoration:none}
.course-header{background:var(--white);border-radius:var(--radius-lg);padding:32px;margin-bottom:24px;box-shadow:var(--shadow)}
.course-header h1{font-size:26px;font-weight:800;margin-bottom:8px;letter-spacing:-.3px}
.course-header .course-desc{font-size:14px;color:var(--gray1);line-height:1.7;margin-bottom:20px}
.course-header .course-stats{display:flex;gap:24px;flex-wrap:wrap}
.course-stat{display:flex;align-items:center;gap:6px;font-size:13px;color:var(--gray1)}
.course-stat .icon{font-size:16px}
.course-progress-block{background:var(--accent-light);border-radius:var(--radius);padding:16px 20px;margin-top:20px;display:flex;align-items:center;gap:16px}
.course-progress-block .cp-bar{flex:1;height:8px;background:rgba(0,0,0,.08);border-radius:4px;overflow:hidden}
.course-progress-block .cp-fill{height:100%;background:var(--accent);border-radius:4px;transition:width .4s}
.course-progress-block .cp-text{font-size:13px;font-weight:600;color:var(--accent-dark);white-space:nowrap}

.section-block{background:var(--white);border-radius:var(--radius-lg);margin-bottom:16px;box-shadow:var(--shadow);overflow:hidden}
.section-header{padding:16px 20px;font-size:14px;font-weight:700;color:var(--dark);display:flex;align-items:center;justify-content:space-between;cursor:pointer;border-bottom:1px solid var(--gray4);user-select:none}
.section-header:hover{background:var(--gray5)}
.section-header .arrow{transition:transform .2s;font-size:12px;color:var(--gray2)}
.section-header.open .arrow{transform:rotate(180deg)}
.section-lessons{display:none}
.section-lessons.open{display:block}
.lesson-row{display:flex;align-items:center;gap:12px;padding:12px 20px;font-size:13px;cursor:pointer;transition:background .1s;border-bottom:1px solid var(--gray5)}
.lesson-row:last-child{border-bottom:none}
.lesson-row:hover{background:var(--primary-light)}
.lesson-row .check{width:22px;height:22px;border-radius:50%;border:2px solid var(--gray3);display:flex;align-items:center;justify-content:center;font-size:12px;flex-shrink:0;transition:all .2s}
.lesson-row .check.done{background:var(--accent);border-color:var(--accent);color:var(--white)}
.lesson-row .lesson-title{flex:1;font-weight:500;color:var(--dark2)}
.lesson-row .lesson-dur{font-size:12px;color:var(--gray2);white-space:nowrap}
.lesson-row .lesson-type{font-size:10px;padding:2px 8px;border-radius:10px;font-weight:600;text-transform:uppercase;letter-spacing:.5px}
.lesson-row .lesson-type.video{background:#fef3e2;color:#e17055}
.lesson-row .lesson-type.reading{background:var(--primary-light);color:var(--primary)}
.lesson-row .lesson-type.exercise{background:var(--accent-light);color:var(--accent-dark)}

/* ── Lesson View ── */
.lesson-layout{display:flex;height:calc(100vh - var(--nav-h))}
.lesson-sidebar{width:var(--sidebar-w);background:var(--white);border-right:1px solid var(--gray4);overflow-y:auto;flex-shrink:0}
.ls-header{padding:16px;border-bottom:1px solid var(--gray4)}
.ls-header .ls-back{font-size:12px;color:var(--gray1);display:flex;align-items:center;gap:4px;margin-bottom:8px;background:none;border:none;padding:0;font-family:var(--font)}
.ls-header .ls-back:hover{color:var(--primary)}
.ls-header h3{font-size:14px;font-weight:700;line-height:1.3}
.ls-section{border-bottom:1px solid var(--gray5)}
.ls-section-title{padding:12px 16px;font-size:12px;font-weight:700;color:var(--gray1);text-transform:uppercase;letter-spacing:.5px;background:var(--gray5)}
.ls-lesson{display:flex;align-items:center;gap:10px;padding:10px 16px;font-size:13px;cursor:pointer;transition:all .1s;border-left:3px solid transparent}
.ls-lesson:hover{background:var(--gray5)}
.ls-lesson.active{background:var(--primary-light);border-left-color:var(--primary);font-weight:600}
.ls-lesson .ls-check{width:18px;height:18px;border-radius:50%;border:2px solid var(--gray3);display:flex;align-items:center;justify-content:center;font-size:10px;flex-shrink:0}
.ls-lesson .ls-check.done{background:var(--accent);border-color:var(--accent);color:var(--white)}
.ls-lesson .ls-title{flex:1;overflow:hidden;text-overflow:ellipsis;white-space:nowrap}

.lesson-content{flex:1;overflow-y:auto;padding:0}
.lesson-top{background:var(--white);padding:24px 32px;border-bottom:1px solid var(--gray4)}
.lesson-top h1{font-size:22px;font-weight:700;margin-bottom:4px}
.lesson-top .lesson-meta{font-size:12px;color:var(--gray2);display:flex;gap:16px}
.lesson-body{padding:32px;max-width:860px}
.lesson-body h2{font-size:18px;font-weight:700;margin:28px 0 12px;color:var(--dark)}
.lesson-body h3{font-size:15px;font-weight:700;margin:20px 0 8px;color:var(--dark2)}
.lesson-body p{margin-bottom:14px;line-height:1.8;color:var(--dark2)}
.lesson-body ul,.lesson-body ol{margin:0 0 14px 24px;line-height:1.8;color:var(--dark2)}
.lesson-body li{margin-bottom:4px}
.lesson-body strong{color:var(--dark)}
.lesson-body code{background:var(--gray5);padding:2px 6px;border-radius:4px;font-size:13px;font-family:'SF Mono',Consolas,monospace}
.lesson-body .formula{background:var(--gray5);border-left:3px solid var(--primary);padding:12px 16px;border-radius:0 var(--radius) var(--radius) 0;margin:16px 0;font-family:'SF Mono',Consolas,monospace;font-size:13px;line-height:1.8;overflow-x:auto}
.lesson-body .callout{background:var(--primary-light);border-radius:var(--radius);padding:14px 18px;margin:16px 0;font-size:13px;line-height:1.7;border-left:3px solid var(--primary)}
.lesson-body .callout.tip{background:var(--accent-light);border-left-color:var(--accent)}
.lesson-body .callout.warning{background:#fef9e7;border-left-color:var(--warning)}

/* ── Video ── */
.video-container{position:relative;padding-bottom:56.25%;height:0;margin:0 0 24px;border-radius:var(--radius-lg);overflow:hidden;background:#000}
.video-container iframe{position:absolute;top:0;left:0;width:100%;height:100%;border:none}
.video-fallback{position:absolute;inset:0;display:flex;flex-direction:column;align-items:center;justify-content:center;background:var(--dark);color:var(--white);gap:12px}
.video-fallback .vf-icon{font-size:48px;opacity:.5}
.video-fallback a{color:var(--accent);font-weight:600;font-size:14px;padding:10px 20px;border:1px solid var(--accent);border-radius:var(--radius);transition:all .15s}
.video-fallback a:hover{background:var(--accent);color:var(--white);text-decoration:none}

/* ── Key Takeaways ── */
.takeaways{background:var(--accent-light);border-radius:var(--radius-lg);padding:20px 24px;margin:24px 0}
.takeaways h3{font-size:14px;font-weight:700;color:var(--accent-dark);margin-bottom:10px;display:flex;align-items:center;gap:6px}
.takeaways ul{margin:0;padding-left:20px}
.takeaways li{font-size:13px;color:var(--dark2);margin-bottom:6px;line-height:1.6}

/* ── Exercise ── */
.exercise-block{background:var(--white);border:2px solid var(--gray4);border-radius:var(--radius-lg);padding:24px;margin:28px 0}
.exercise-block h3{font-size:16px;font-weight:700;margin-bottom:4px;display:flex;align-items:center;gap:8px}
.exercise-block .ex-type{font-size:10px;padding:3px 8px;border-radius:10px;background:var(--purple);color:var(--white);font-weight:600;text-transform:uppercase;letter-spacing:.5px}
.exercise-block .ex-question{font-size:14px;line-height:1.7;margin:12px 0 16px;color:var(--dark2)}
.ex-options{display:flex;flex-direction:column;gap:8px;margin-bottom:16px}
.ex-option{display:flex;align-items:center;gap:10px;padding:12px 16px;border:2px solid var(--gray4);border-radius:var(--radius);cursor:pointer;transition:all .15s;font-size:13px}
.ex-option:hover{border-color:var(--primary);background:var(--primary-light)}
.ex-option.selected{border-color:var(--primary);background:var(--primary-light);font-weight:600}
.ex-option.correct{border-color:var(--accent);background:var(--accent-light)}
.ex-option.incorrect{border-color:var(--danger);background:#fef0ed}
.ex-option .radio{width:18px;height:18px;border-radius:50%;border:2px solid var(--gray3);flex-shrink:0;display:flex;align-items:center;justify-content:center;transition:all .15s}
.ex-option.selected .radio{border-color:var(--primary);background:var(--primary)}
.ex-option.selected .radio::after{content:'';width:6px;height:6px;border-radius:50%;background:var(--white)}
.ex-option.correct .radio{border-color:var(--accent);background:var(--accent)}
.ex-option.correct .radio::after{content:'✓';color:var(--white);font-size:11px;font-weight:700}
.ex-calc-input{display:flex;gap:8px;align-items:center;margin-bottom:16px}
.ex-calc-input input{padding:10px 14px;border:2px solid var(--gray4);border-radius:var(--radius);font-size:14px;font-family:var(--font);width:200px;outline:none;transition:border-color .15s}
.ex-calc-input input:focus{border-color:var(--primary)}
.ex-calc-input .unit{font-size:13px;color:var(--gray1)}
.ex-submit{padding:10px 24px;background:var(--primary);color:var(--white);border:none;border-radius:var(--radius);font-size:13px;font-weight:600;transition:all .15s}
.ex-submit:hover{background:var(--primary-dark)}
.ex-submit:disabled{opacity:.5;cursor:not-allowed}
.ex-explanation{margin-top:16px;padding:16px;border-radius:var(--radius);font-size:13px;line-height:1.7;display:none}
.ex-explanation.show{display:block}
.ex-explanation.correct{background:var(--accent-light);border:1px solid var(--accent);color:var(--accent-dark)}
.ex-explanation.incorrect{background:#fef0ed;border:1px solid var(--danger);color:#c0392b}

/* ── Lesson Navigation ── */
.lesson-nav{display:flex;justify-content:space-between;align-items:center;padding:24px 0;margin-top:32px;border-top:1px solid var(--gray4)}
.lesson-nav-btn{display:flex;align-items:center;gap:8px;padding:10px 20px;border-radius:var(--radius);font-size:13px;font-weight:600;border:2px solid var(--gray4);background:var(--white);color:var(--dark);transition:all .15s}
.lesson-nav-btn:hover{border-color:var(--primary);color:var(--primary)}
.lesson-nav-btn.primary{background:var(--primary);color:var(--white);border-color:var(--primary)}
.lesson-nav-btn.primary:hover{background:var(--primary-dark);border-color:var(--primary-dark)}
.mark-complete-btn{padding:10px 20px;border-radius:var(--radius);font-size:13px;font-weight:600;border:2px solid var(--accent);background:var(--accent);color:var(--white);transition:all .15s}
.mark-complete-btn:hover{background:var(--accent-dark);border-color:var(--accent-dark)}
.mark-complete-btn.completed{background:var(--white);color:var(--accent);cursor:default}

/* ── Resources Page ── */
.resources-page{max-width:900px;margin:0 auto;padding:32px 24px 64px}
.resource-grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(260px,1fr));gap:16px;margin-top:16px}
.resource-card{background:var(--white);border-radius:var(--radius);padding:20px;box-shadow:var(--shadow);transition:all .15s}
.resource-card:hover{box-shadow:var(--shadow-lg);transform:translateY(-2px)}
.resource-card h4{font-size:14px;font-weight:700;margin-bottom:4px}
.resource-card p{font-size:12px;color:var(--gray1);margin-bottom:12px;line-height:1.5}
.resource-card a{font-size:12px;font-weight:600;color:var(--primary)}

/* ── Mobile Sidebar Toggle ── */
.sidebar-toggle{display:none;position:fixed;bottom:20px;left:20px;z-index:50;width:44px;height:44px;border-radius:50%;background:var(--primary);color:var(--white);border:none;font-size:18px;box-shadow:var(--shadow-lg);align-items:center;justify-content:center}

/* ── Responsive ── */
/* ── Auth Page ── */
.auth-page{display:flex;align-items:center;justify-content:center;min-height:100vh;background:linear-gradient(135deg,#1c1d1f 0%,#2d2f31 100%);padding:20px}
.auth-card{background:var(--white);border-radius:var(--radius-lg);padding:40px;width:100%;max-width:420px;box-shadow:var(--shadow-lg)}
.auth-logo{font-size:24px;font-weight:800;text-align:center;margin-bottom:4px;color:var(--dark)}
.auth-logo span{color:var(--accent)}
.auth-subtitle{font-size:12px;text-align:center;color:var(--gray2);margin-bottom:28px;letter-spacing:.5px}
.auth-tabs{display:flex;border-bottom:2px solid var(--gray4);margin-bottom:24px}
.auth-tab{flex:1;padding:10px;text-align:center;font-size:14px;font-weight:600;color:var(--gray2);background:none;border:none;border-bottom:2px solid transparent;margin-bottom:-2px;transition:all .15s}
.auth-tab:hover{color:var(--dark)}
.auth-tab.active{color:var(--primary);border-bottom-color:var(--primary)}
.auth-form{display:flex;flex-direction:column;gap:16px}
.auth-field label{display:block;font-size:12px;font-weight:600;color:var(--dark2);margin-bottom:6px}
.auth-field input{width:100%;padding:10px 14px;border:2px solid var(--gray4);border-radius:var(--radius);font-size:14px;font-family:var(--font);outline:none;transition:border-color .15s}
.auth-field input:focus{border-color:var(--primary)}
.auth-error{font-size:13px;color:var(--danger);min-height:20px}
.auth-submit{padding:12px;background:var(--primary);color:var(--white);border:none;border-radius:var(--radius);font-size:14px;font-weight:600;font-family:var(--font);transition:all .15s}
.auth-submit:hover{background:var(--primary-dark)}
.auth-submit:disabled{opacity:.6;cursor:not-allowed}

/* ── Nav User ── */
.nav-user{display:flex;align-items:center;gap:12px;margin-left:16px}
.nav-username{color:var(--gray2);font-size:13px;font-weight:500}
.nav-logout{background:none;border:1px solid rgba(255,255,255,.2);color:var(--gray2);font-size:12px;padding:5px 12px;border-radius:6px;font-family:var(--font);transition:all .15s}
.nav-logout:hover{border-color:rgba(255,255,255,.5);color:var(--white)}

/* ── Responsive ── */
@media(max-width:768px){
  .hero{padding:28px 20px}
  .hero h1{font-size:22px}
  .hero-stats{gap:20px}
  .hero-stat .val{font-size:18px}
  .course-grid{grid-template-columns:1fr}
  .lesson-layout{flex-direction:column}
  .lesson-sidebar{width:100%;max-height:0;overflow:hidden;transition:max-height .3s;border-right:none;border-bottom:1px solid var(--gray4)}
  .lesson-sidebar.mobile-open{max-height:60vh}
  .sidebar-toggle{display:flex}
  .lesson-body{padding:20px}
  .lesson-top{padding:16px 20px}
  .lesson-top h1{font-size:18px}
  .nav-links{display:none}
  .nav-progress{display:none}
  .nav-user{margin-left:auto}
  .nav-username{display:none}
  .course-detail{padding:20px 16px 64px}
  .course-header{padding:20px}
  .course-header h1{font-size:20px}
  .auth-card{padding:28px 20px}
}

/* ─── Code blocks (added for Python / AI lessons) ─── */
.lesson-body pre{background:var(--gray5);border-left:3px solid var(--accent);border-radius:0 var(--radius) var(--radius) 0;padding:14px 16px;margin:16px 0;overflow-x:auto;font-family:'SF Mono',Consolas,monospace;font-size:12.5px;line-height:1.7;white-space:pre}
.lesson-body pre code{background:none;padding:0;font-size:inherit;color:var(--ink,inherit)}
.lesson-body .skill-tags{display:flex;flex-wrap:wrap;gap:8px;margin:14px 0}
.lesson-body .skill-tag{background:var(--primary-light);color:var(--primary);border-radius:999px;padding:4px 12px;font-size:12px;font-weight:600}

/* ─── Practice: dataset block & "Practice More" drills ─── */
.dataset-block{background:linear-gradient(135deg,var(--primary-light),#fff);border:1px solid var(--primary);border-radius:var(--radius);padding:18px 20px;margin:24px 0}
.dataset-block h3{font-size:15px;font-weight:700;margin-bottom:6px;color:var(--primary)}
.dataset-block .dataset-title{font-weight:600;font-size:13px;margin-bottom:4px}
.dataset-block .dataset-task{font-size:13px;line-height:1.6;color:var(--gray1);margin-bottom:14px}
.dataset-download{display:inline-block;background:var(--primary);color:#fff;text-decoration:none;font-size:13px;font-weight:600;padding:9px 16px;border-radius:8px;transition:opacity .15s}
.dataset-download:hover{opacity:.88}
.practice-more{margin:28px 0 8px}
.practice-more-header{display:flex;align-items:baseline;gap:12px;border-top:2px dashed var(--gray4);padding-top:20px;margin-bottom:4px}
.practice-more-header h3{font-size:17px;font-weight:800}
.practice-more-header span{font-size:12px;color:var(--gray1)}
.exercise-block.drill{margin-top:16px}

/* ─── Module resources embedded in course page & lessons ─── */
.lesson-resources{margin:24px 0}
.lesson-resources h3{font-size:15px;font-weight:700;margin-bottom:12px}
.lr-grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(220px,1fr));gap:12px}
.lr-card{display:flex;flex-direction:column;gap:4px;text-decoration:none;background:var(--card,#fff);border:1px solid var(--gray4);border-radius:var(--radius);padding:14px 16px;transition:border-color .15s,transform .15s,box-shadow .15s}
.lr-card:hover{border-color:var(--primary);transform:translateY(-2px);box-shadow:0 4px 14px rgba(0,0,0,.06)}
.lr-name{font-weight:600;font-size:13px;color:var(--primary)}
.lr-desc{font-size:12px;line-height:1.5;color:var(--gray1)}
.course-detail .lesson-resources{margin-top:28px;border-top:1px solid var(--gray4);padding-top:24px}