/* ===== RESET & BASE ===== */
*,*::before,*::after{margin:0;padding:0;box-sizing:border-box}
:root{
  --bg:#0a0a0f;--bg-secondary:#111118;--bg-card:#16161f;--bg-card-hover:#1c1c28;
  --accent:#6c5ce7;--accent-light:#a29bfe;--accent-glow:rgba(108,92,231,.25);
  --text:#f0f0f5;--text-sec:#a0a0b8;--text-muted:#6c6c80;
  --border:rgba(255,255,255,.06);--border-hover:rgba(255,255,255,.12);
  --gradient:linear-gradient(135deg,#6c5ce7,#a29bfe);
  --radius:12px;--radius-sm:8px;
  --sidebar-w:280px;--header-h:60px;
  --font:-apple-system,BlinkMacSystemFont,'Segoe UI','Inter',sans-serif;
  --mono:'JetBrains Mono','Fira Code','Cascadia Code',monospace;
  --green:#00d68f;--yellow:#ffaa00;--red:#ff4757;--blue:#339af0;
  --tip-bg:rgba(0,214,143,.08);--tip-border:#00d68f;
  --warn-bg:rgba(255,170,0,.08);--warn-border:#ffaa00;
  --imp-bg:rgba(108,92,231,.08);--imp-border:#6c5ce7;
}
html{scroll-behavior:smooth;-webkit-font-smoothing:antialiased}
body{font-family:var(--font);background:var(--bg);color:var(--text);line-height:1.7;overflow-x:hidden}
a{text-decoration:none;color:inherit}
ul,ol{list-style:none}
img{max-width:100%;display:block}

/* ===== HEADER ===== */
.header{position:fixed;top:0;left:0;right:0;height:var(--header-h);background:rgba(10,10,15,.92);backdrop-filter:blur(20px);-webkit-backdrop-filter:blur(20px);border-bottom:1px solid var(--border);z-index:100;display:flex;align-items:center}
.header .container{display:flex;align-items:center;justify-content:space-between;width:100%;max-width:1440px;margin:0 auto;padding:0 24px}
.header-logo{font-size:1.3rem;font-weight:800;background:var(--gradient);-webkit-background-clip:text;-webkit-text-fill-color:transparent;background-clip:text}
.header-nav{display:flex;align-items:center;gap:24px}
.header-nav a{font-size:.88rem;font-weight:500;color:var(--text-sec);transition:color .2s}
.header-nav a:hover{color:var(--text)}
.header-nav .btn-sm{padding:7px 18px;background:var(--gradient);color:#fff;border-radius:var(--radius-sm);font-weight:600;font-size:.85rem;transition:opacity .2s}
.header-nav .btn-sm:hover{opacity:.85}
.mobile-toggle{display:none;background:none;border:none;cursor:pointer;flex-direction:column;gap:5px;padding:4px}
.mobile-toggle span{width:22px;height:2px;background:var(--text);border-radius:2px;transition:transform .3s,opacity .3s}
.mobile-toggle.active span:nth-child(1){transform:rotate(45deg) translate(5px,5px)}
.mobile-toggle.active span:nth-child(2){opacity:0}
.mobile-toggle.active span:nth-child(3){transform:rotate(-45deg) translate(5px,-5px)}

/* ===== LAYOUT ===== */
.page-wrapper{display:flex;margin-top:var(--header-h);min-height:calc(100vh - var(--header-h))}
.sidebar{position:fixed;top:var(--header-h);left:0;bottom:0;width:var(--sidebar-w);background:var(--bg-secondary);border-right:1px solid var(--border);overflow-y:auto;padding:24px 0;z-index:50;transition:transform .3s}
.sidebar::-webkit-scrollbar{width:4px}
.sidebar::-webkit-scrollbar-thumb{background:var(--border-hover);border-radius:4px}
.sidebar-section{margin-bottom:8px}
.sidebar-heading{padding:10px 20px 6px;font-size:.7rem;font-weight:700;text-transform:uppercase;letter-spacing:1.5px;color:var(--text-muted);cursor:pointer;display:flex;align-items:center;justify-content:space-between;user-select:none}
.sidebar-heading::after{content:'▾';font-size:.65rem;transition:transform .2s}
.sidebar-heading.collapsed::after{transform:rotate(-90deg)}
.sidebar-items{overflow:hidden;transition:max-height .3s ease}
.sidebar-items.collapsed{max-height:0!important}
.sidebar-items a{display:flex;align-items:center;gap:10px;padding:8px 20px 8px 28px;font-size:.87rem;color:var(--text-sec);transition:background .2s,color .2s;border-left:2px solid transparent}
.sidebar-items a:hover{background:rgba(255,255,255,.03);color:var(--text)}
.sidebar-items a.active{color:var(--accent-light);border-left-color:var(--accent);background:rgba(108,92,231,.06);font-weight:600}
.sidebar-items a .num{font-size:.75rem;font-weight:600;color:var(--text-muted);min-width:18px}

/* ===== MAIN CONTENT ===== */
.main{margin-left:var(--sidebar-w);flex:1;padding:48px 56px 80px;max-width:900px}
.main>*+*{margin-top:28px}

/* ===== TYPOGRAPHY ===== */
h1{font-size:2.2rem;font-weight:700;letter-spacing:-.02em;line-height:1.2}
h2{font-size:1.5rem;font-weight:700;letter-spacing:-.01em;margin-top:48px;margin-bottom:16px;padding-bottom:10px;border-bottom:1px solid var(--border)}
h3{font-size:1.15rem;font-weight:600;margin-top:32px;margin-bottom:10px}
h4{font-size:1rem;font-weight:600;margin-top:24px;margin-bottom:8px}
p{margin-bottom:14px;color:var(--text-sec);line-height:1.8}
strong{color:var(--text);font-weight:600}
.subtitle{font-size:1.05rem;color:var(--text-sec);margin-top:8px;margin-bottom:32px;line-height:1.7}

/* ===== TOPIC META ===== */
.topic-meta{display:flex;gap:12px;flex-wrap:wrap;margin-bottom:32px}
.topic-badge{padding:4px 14px;border-radius:20px;font-size:.78rem;font-weight:600}
.badge-beginner{background:rgba(0,214,143,.1);color:var(--green)}
.badge-intermediate{background:rgba(255,170,0,.1);color:var(--yellow)}
.badge-advanced{background:rgba(255,71,87,.1);color:var(--red)}
.badge-hands-on{background:rgba(51,154,240,.1);color:var(--blue)}

/* ===== CODE BLOCKS ===== */
.code-block{position:relative;background:#0d0d14;border:1px solid var(--border);border-radius:var(--radius);margin:16px 0 20px;overflow:hidden}
.code-header{display:flex;align-items:center;justify-content:space-between;padding:8px 16px;background:rgba(255,255,255,.03);border-bottom:1px solid var(--border);font-size:.78rem;color:var(--text-muted)}
.code-header .lang{font-weight:600;text-transform:uppercase;letter-spacing:.5px}
.copy-btn{background:rgba(255,255,255,.06);border:1px solid var(--border);color:var(--text-sec);padding:4px 12px;border-radius:6px;cursor:pointer;font-size:.75rem;font-family:var(--font);transition:background .2s,color .2s}
.copy-btn:hover{background:rgba(255,255,255,.1);color:var(--text)}
.copy-btn.copied{background:rgba(0,214,143,.15);color:var(--green);border-color:rgba(0,214,143,.3)}
pre{padding:16px 20px;overflow-x:auto;font-family:var(--mono);font-size:.88rem;line-height:1.7;color:#c9d1d9}
pre::-webkit-scrollbar{height:6px}
pre::-webkit-scrollbar-thumb{background:var(--border-hover);border-radius:3px}
code{font-family:var(--mono);font-size:.88rem}
p code,li code{background:rgba(108,92,231,.12);padding:2px 7px;border-radius:4px;font-size:.84rem;color:var(--accent-light)}

/* ===== INFO BOXES ===== */
.info-box{border-radius:var(--radius);padding:16px 20px;margin:20px 0;border-left:4px solid;display:flex;gap:12px}
.info-box .icon{font-size:1.2rem;flex-shrink:0;margin-top:2px}
.info-box .content{flex:1}
.info-box .content p{margin-bottom:6px;color:var(--text-sec)}
.info-box .content p:last-child{margin-bottom:0}
.info-box .label{font-weight:700;font-size:.85rem;margin-bottom:4px}
.info-tip{background:var(--tip-bg);border-color:var(--tip-border)}
.info-tip .label{color:var(--green)}
.info-warning{background:var(--warn-bg);border-color:var(--warn-border)}
.info-warning .label{color:var(--yellow)}
.info-important{background:var(--imp-bg);border-color:var(--imp-border)}
.info-important .label{color:var(--accent-light)}

/* ===== VISUAL / DIAGRAM ===== */
.diagram{background:var(--bg-card);border:1px solid var(--border);border-radius:var(--radius);padding:32px;margin:20px 0;text-align:center;overflow-x:auto}
.diagram-title{font-size:.8rem;font-weight:600;text-transform:uppercase;letter-spacing:1px;color:var(--text-muted);margin-bottom:20px}
.diagram-row{display:flex;align-items:center;justify-content:center;gap:12px;flex-wrap:wrap;margin:12px 0}
.diagram-box{padding:12px 20px;background:var(--bg-card-hover);border:1px solid var(--border-hover);border-radius:var(--radius-sm);font-size:.88rem;font-weight:500;color:var(--text);min-width:120px;text-align:center}
.diagram-box.highlight{border-color:var(--accent);background:rgba(108,92,231,.1);color:var(--accent-light)}
.diagram-arrow{color:var(--text-muted);font-size:1.2rem}
.diagram-group{display:flex;flex-direction:column;gap:8px;padding:16px;border:1px dashed var(--border-hover);border-radius:var(--radius);min-width:140px}
.diagram-group-title{font-size:.75rem;font-weight:600;text-transform:uppercase;letter-spacing:1px;color:var(--accent-light);margin-bottom:4px}
.diagram-small{padding:8px 14px;background:var(--bg);border:1px solid var(--border);border-radius:6px;font-size:.8rem;color:var(--text-sec)}

/* ===== INTERVIEW SECTION ===== */
.interview-section{margin:24px 0}
.interview-section h3{margin-bottom:16px}
.question-list{display:flex;flex-direction:column;gap:12px}
.question{background:var(--bg-card);border:1px solid var(--border);border-radius:var(--radius);overflow:hidden}
.question-header{padding:14px 20px;cursor:pointer;display:flex;align-items:center;justify-content:space-between;font-size:.92rem;font-weight:500;transition:background .2s;user-select:none}
.question-header:hover{background:rgba(255,255,255,.02)}
.question-header .q-icon{font-size:.75rem;color:var(--text-muted);transition:transform .25s}
.question-header .q-icon.open{transform:rotate(180deg)}
.question-answer{max-height:0;overflow:hidden;transition:max-height .35s ease,padding .35s ease}
.question-answer.open{max-height:600px;padding:0 20px 16px}
.question-answer p{font-size:.9rem;color:var(--text-sec);line-height:1.7}

/* ===== BOTTOM NAV ===== */
.bottom-nav{display:flex;align-items:center;justify-content:space-between;margin-top:64px;padding-top:32px;border-top:1px solid var(--border);gap:16px}
.nav-btn{display:flex;flex-direction:column;gap:4px;padding:16px 24px;background:var(--bg-card);border:1px solid var(--border);border-radius:var(--radius);transition:border-color .2s,background .2s,transform .2s;max-width:45%;min-width:0}
.nav-btn:hover{border-color:var(--border-hover);background:var(--bg-card-hover);transform:translateY(-2px)}
.nav-btn .dir{font-size:.75rem;font-weight:600;text-transform:uppercase;letter-spacing:1px;color:var(--text-muted)}
.nav-btn .title{font-size:.92rem;font-weight:500;color:var(--text);white-space:nowrap;overflow:hidden;text-overflow:ellipsis}
.nav-btn.prev{align-items:flex-start}
.nav-btn.next{align-items:flex-end;margin-left:auto}
.back-link{display:inline-flex;align-items:center;gap:6px;font-size:.85rem;color:var(--accent-light);margin-top:16px;transition:gap .2s}
.back-link:hover{gap:10px}

/* ===== INDEX PAGE ===== */
.course-hero{text-align:center;padding:64px 0 48px}
.course-hero h1{font-size:clamp(2rem,5vw,3rem);margin-bottom:12px}
.course-hero .subtitle{max-width:600px;margin:12px auto 32px}
.course-stats{display:flex;justify-content:center;gap:40px;flex-wrap:wrap;margin-top:32px}
.course-stat{text-align:center}
.course-stat .val{font-size:1.6rem;font-weight:700;font-family:var(--mono);color:var(--text)}
.course-stat .lbl{font-size:.8rem;color:var(--text-muted);margin-top:2px}
.btn-start{display:inline-flex;align-items:center;gap:8px;padding:14px 36px;background:var(--gradient);color:#fff;border-radius:var(--radius-sm);font-weight:600;font-size:1rem;transition:transform .2s,box-shadow .2s;border:none;cursor:pointer}
.btn-start:hover{transform:translateY(-2px);box-shadow:0 8px 32px var(--accent-glow)}
.path-grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(280px,1fr));gap:20px;margin-top:48px}
.path-card{background:var(--bg-card);border:1px solid var(--border);border-radius:var(--radius);padding:28px 24px;transition:transform .3s,border-color .3s,box-shadow .3s}
.path-card:hover{transform:translateY(-3px);border-color:var(--border-hover);box-shadow:0 4px 24px rgba(0,0,0,.3)}
.path-card .card-icon{font-size:1.8rem;margin-bottom:14px}
.path-card h3{font-size:1.1rem;font-weight:600;margin:0 0 8px;border:none;padding:0}
.path-card p{font-size:.88rem;color:var(--text-sec);margin-bottom:14px}
.path-card .topics{display:flex;flex-direction:column;gap:6px;margin-top:12px;padding-top:12px;border-top:1px solid var(--border)}
.path-card .topics a{font-size:.85rem;color:var(--text-sec);padding:4px 0;display:flex;align-items:center;gap:8px;transition:color .2s}
.path-card .topics a:hover{color:var(--accent-light)}
.path-card .topics a .num{font-size:.75rem;color:var(--text-muted);min-width:20px;font-weight:600}

/* ===== PROGRESS BAR ===== */
.progress-wrap{margin:32px 0}
.progress-bar{height:6px;background:var(--bg-card);border-radius:3px;overflow:hidden}
.progress-fill{height:100%;background:var(--gradient);border-radius:3px;width:0;transition:width .5s ease}
.progress-label{font-size:.8rem;color:var(--text-muted);margin-top:6px;text-align:right}

/* ===== ORDERED LISTS in content ===== */
.main ol{list-style:decimal;padding-left:24px;margin:12px 0}
.main ol li{margin-bottom:8px;color:var(--text-sec);font-size:.95rem}
.main ul.bullet{list-style:disc;padding-left:24px;margin:12px 0}
.main ul.bullet li{margin-bottom:8px;color:var(--text-sec);font-size:.95rem}

/* ===== TABLE ===== */
table{width:100%;border-collapse:collapse;margin:16px 0;font-size:.9rem}
th,td{padding:10px 16px;text-align:left;border-bottom:1px solid var(--border)}
th{font-weight:600;color:var(--text);background:var(--bg-card);font-size:.82rem;text-transform:uppercase;letter-spacing:.5px}
td{color:var(--text-sec)}
tr:hover td{background:rgba(255,255,255,.02)}

/* ===== RESPONSIVE ===== */
@media(max-width:900px){
  .sidebar{transform:translateX(-100%)}
  .sidebar.open{transform:translateX(0)}
  .main{margin-left:0;padding:32px 20px 60px}
  .mobile-toggle{display:flex}
  .header-nav .nav-link{display:none}
  .course-stats{gap:24px}
  .bottom-nav{flex-direction:column}
  .nav-btn{max-width:100%;width:100%}
  .nav-btn.next{align-items:flex-start}
}
@media(max-width:480px){
  .main{padding:24px 16px 48px}
  h1{font-size:1.6rem}
  h2{font-size:1.25rem}
  .diagram-row{flex-direction:column}
}