/* Shell Nav (injected) */
.shell-nav { position:fixed;top:0;left:0;right:0;z-index:2000;background:rgba(26,26,46,0.85);backdrop-filter:blur(24px);-webkit-backdrop-filter:blur(24px);border-bottom:1px solid rgba(255,255,255,0.06);transition:all 0.4s cubic-bezier(0.16,1,0.3,1); }
.shell-nav.scrolled { background:rgba(15,15,26,0.96);box-shadow:0 4px 30px rgba(0,0,0,0.5); }
.nav-inner { max-width:1280px;margin:0 auto;display:flex;align-items:center;justify-content:space-between;padding:0 32px;height:64px; }
.nav-brand { display:flex;align-items:center;gap:12px;text-decoration:none;font-weight:700;font-size:17px;color:#f0f0f0; }
.nav-brand .brand-icon { width:34px;height:34px;background:linear-gradient(135deg,#7DB6B1,#5a9e98);border-radius:10px;display:flex;align-items:center;justify-content:center;font-size:15px;font-weight:900;color:#0f0f1a; }
.nav-brand span { color:#7DB6B1; }
.nav-links { display:flex;align-items:center;gap:4px; }
.nav-link { padding:8px 16px;border-radius:8px;text-decoration:none;font-size:14px;font-weight:600;color:#9ca3af; }
.nav-link:hover { color:#f0f0f0;background:rgba(255,255,255,0.04); }
.nav-cta { padding:8px 24px;border-radius:8px;background:linear-gradient(135deg,#7DB6B1,#5a9e98);color:#0f0f1a;text-decoration:none;font-size:14px;font-weight:700; }
.nav-cta:hover { transform:translateY(-1px);box-shadow:0 4px 20px rgba(125,182,177,0.25); }
.nav-progress { position:absolute;bottom:-2px;left:0;height:2px;background:linear-gradient(90deg,#7DB6B1,#a3d4cf);width:0%;transition:width 0.1s linear; }
.breadcrumbs { display:flex;align-items:center;gap:8px;padding:80px 32px 8px;max-width:1280px;margin:0 auto;font-size:13px;color:#6b7280; }
.breadcrumbs a { color:#9ca3af;text-decoration:none; }
.breadcrumbs a:hover { color:#7DB6B1; }
.breadcrumbs .current { color:#f0f0f0;font-weight:600; }
.breadcrumbs .separator { margin:0 2px; }

:root {
            --primary: #2563eb;
            --primary-dark: #1d4ed8;
            --primary-light: #3b82f6;
            --accent: #8b5cf6;
            --success: #10b981;
            --warning: #f59e0b;
            --error: #ef4444;
            --info: #06b6d4;
            --bg: #f0f4ff;
            --bg-card: #ffffff;
            --bg-nav: rgba(255,255,255,0.92);
            --text: #1e293b;
            --text-secondary: #475569;
            --text-muted: #94a3b8;
            --border: #e2e8f0;
            --shadow: 0 4px 24px rgba(0,0,0,0.08);
            --shadow-lg: 0 12px 40px rgba(0,0,0,0.12);
            --radius: 16px;
            --radius-sm: 10px;
            --transition: 0.3s cubic-bezier(0.4, 0, 0.2, 1);
        }

        [data-theme="dark"] {
            --bg: #0f172a;
            --bg-card: #1e293b;
            --bg-nav: rgba(15,23,42,0.95);
            --text: #e2e8f0;
            --text-secondary: #94a3b8;
            --text-muted: #64748b;
            --border: #334155;
            --shadow: 0 4px 24px rgba(0,0,0,0.3);
            --shadow-lg: 0 12px 40px rgba(0,0,0,0.4);
        }

        * { box-sizing: border-box; margin: 0; padding: 0; }

        body {
            font-family: 'Inter', system-ui, sans-serif;
            line-height: 1.7;
            color: var(--text);
            background: var(--bg);
            min-height: 100vh;
            transition: background var(--transition), color var(--transition);
        }

        /* ─── Navigation ─── */
        .navbar {
            position: sticky; top: 64px; z-index: 10;
            background: var(--bg-nav);
            backdrop-filter: blur(20px);
            border-bottom: 1px solid var(--border);
            padding: 0 24px;
            transition: background var(--transition);
        }
        .nav-inner {
            max-width: 1200px; margin: 0 auto;
            display: flex; align-items: center; justify-content: space-between;
            height: 60px; gap: 16px;
        }
        .nav-brand {
            font-weight: 700; font-size: 1rem; color: var(--primary);
            white-space: nowrap; flex-shrink: 0;
        }
        .nav-links {
            display: flex; gap: 4px; overflow-x: auto;
            scrollbar-width: none; flex: 1; justify-content: center;
        }
        .nav-links::-webkit-scrollbar { display: none; }
        .nav-link {
            padding: 6px 14px; border-radius: 8px; font-size: 0.8rem;
            color: var(--text-secondary); text-decoration: none;
            white-space: nowrap; transition: all var(--transition); font-weight: 500;
        }
        .nav-link:hover, .nav-link.active {
            background: var(--primary); color: #fff;
        }
        .theme-toggle {
            width: 42px; height: 42px; border-radius: 12px; border: 2px solid var(--border);
            background: var(--bg-card); cursor: pointer; font-size: 1.2rem;
            display: flex; align-items: center; justify-content: center;
            transition: all var(--transition); flex-shrink: 0;
        }
        .theme-toggle:hover { border-color: var(--primary); transform: scale(1.05); }

        /* ─── Layout ─── */
        .container { max-width: 1000px; margin: 0 auto; padding: 24px 20px 60px; }

        .header {
            text-align: center; padding: 48px 20px 32px;
            animation: fadeUp 0.8s ease-out;
        }
        .header h1 {
            font-size: clamp(1.8rem, 4vw, 2.8rem); font-weight: 700;
            background: linear-gradient(135deg, var(--primary), var(--accent));
            -webkit-background-clip: text; -webkit-text-fill-color: transparent;
            background-clip: text; margin-bottom: 8px;
        }
        .header p { color: var(--text-secondary); font-size: 1.05rem; }

        /* ─── Cards ─── */
        .card {
            background: var(--bg-card); border-radius: var(--radius);
            box-shadow: var(--shadow); padding: 32px;
            margin-bottom: 28px; border: 1px solid var(--border);
            transition: all var(--transition);
            opacity: 0; transform: translateY(24px);
        }
        .card.visible { opacity: 1; transform: translateY(0); }
        .card:hover { box-shadow: var(--shadow-lg); }

        h2 {
            font-size: 1.45rem; font-weight: 700; color: var(--primary);
            margin-bottom: 20px; display: flex; align-items: center; gap: 12px;
        }
        h2 .icon {
            width: 40px; height: 40px; border-radius: 12px;
            display: flex; align-items: center; justify-content: center;
            font-size: 1.2rem; flex-shrink: 0;
            background: linear-gradient(135deg, var(--primary), var(--accent));
        }
        h3 { font-size: 1.15rem; font-weight: 600; color: var(--text); margin: 20px 0 12px; }
        h4 { font-size: 1rem; font-weight: 600; color: var(--primary-dark); margin-bottom: 8px; }
        p { margin-bottom: 12px; color: var(--text-secondary); }
        ul { margin: 0 0 12px 20px; color: var(--text-secondary); }
        li { margin-bottom: 6px; }

        /* ─── Special Boxes ─── */
        .outcomes {
            background: linear-gradient(135deg, #eff6ff, #dbeafe);
            border: 1px solid #93c5fd; border-radius: var(--radius-sm);
            padding: 20px; margin-bottom: 20px;
        }
        [data-theme="dark"] .outcomes {
            background: linear-gradient(135deg, #1e3a5f, #1e293b);
            border-color: #2563eb55;
        }
        .formula-box {
            background: linear-gradient(135deg, var(--primary), var(--accent));
            color: #fff; padding: 20px; border-radius: var(--radius-sm);
            text-align: center; margin: 20px 0; font-size: 1.1rem;
            box-shadow: 0 4px 16px rgba(37,99,235,0.3);
        }
        .formula-box p { color: #fff; margin-bottom: 6px; }
        .formula-box ul { list-style: none; margin: 0; padding: 0; }
        .formula-box li { margin-bottom: 2px; color: rgba(255,255,255,0.9); }

        .example-box {
            background: linear-gradient(135deg, #fefce8, #fef3c7);
            border: 1px solid #fbbf24; border-radius: var(--radius-sm);
            padding: 20px; margin: 16px 0;
        }
        [data-theme="dark"] .example-box {
            background: linear-gradient(135deg, #422006, #1e293b);
            border-color: #b4590055;
        }
        .practical-box {
            background: linear-gradient(135deg, #ecfdf5, #d1fae5);
            border: 1px solid #34d399; border-radius: var(--radius-sm);
            padding: 20px; margin: 16px 0;
        }
        [data-theme="dark"] .practical-box {
            background: linear-gradient(135deg, #064e3b33, #1e293b);
            border-color: #10b98155;
        }
        .warning-box {
            background: linear-gradient(135deg, #fef2f2, #fee2e2);
            border: 1px solid #f87171; border-radius: var(--radius-sm);
            padding: 16px; margin: 12px 0;
        }
        [data-theme="dark"] .warning-box {
            background: linear-gradient(135deg, #450a0a33, #1e293b);
            border-color: #ef444455;
        }
        .explanation {
            font-style: italic; color: var(--text-muted);
            padding: 12px 16px; border-left: 3px solid var(--border);
            margin: 12px 0; border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
        }

        /* ─── Simulation Containers ─── */
        .sim-container {
            background: var(--bg); border: 2px solid var(--border);
            border-radius: var(--radius); padding: 24px;
            margin: 20px 0; transition: all var(--transition);
        }
        .sim-container:hover { border-color: var(--primary); }
        .sim-canvas-wrap {
            position: relative; width: 100%; margin-bottom: 16px;
            border-radius: var(--radius-sm); overflow: hidden;
            background: #fff; border: 1px solid var(--border);
        }
        [data-theme="dark"] .sim-canvas-wrap { background: #0f172a; }
        .sim-canvas-wrap canvas { display: block; width: 100%; height: auto; }

        .sim-controls {
            display: flex; flex-wrap: wrap; gap: 12px;
            align-items: center; margin-bottom: 16px;
        }
        .sim-readout {
            display: grid; grid-template-columns: repeat(auto-fit, minmax(140px, 1fr));
            gap: 10px; margin-top: 12px;
        }
        .readout-item {
            background: var(--bg-card); border: 1px solid var(--border);
            border-radius: var(--radius-sm); padding: 10px 14px; text-align: center;
        }
        .readout-label { font-size: 0.75rem; color: var(--text-muted); font-weight: 500; text-transform: uppercase; letter-spacing: 0.5px; }
        .readout-value { font-size: 1.3rem; font-weight: 700; color: var(--primary); margin-top: 2px; }

        .slider-group { flex: 1; min-width: 180px; }
        .slider-group label {
            display: flex; justify-content: space-between; align-items: center;
            font-size: 0.85rem; font-weight: 500; color: var(--text-secondary); margin-bottom: 4px;
        }
        .slider-group label span { color: var(--primary); font-weight: 700; }
        input[type="range"] {
            width: 100%; height: 6px; -webkit-appearance: none; appearance: none;
            background: var(--border); border-radius: 3px; outline: none;
        }
        input[type="range"]::-webkit-slider-thumb {
            -webkit-appearance: none; appearance: none;
            width: 20px; height: 20px; border-radius: 50%;
            background: var(--primary); cursor: pointer; border: 3px solid #fff;
            box-shadow: 0 2px 6px rgba(0,0,0,0.2);
        }

        /* ─── Buttons ─── */
        .btn {
            padding: 10px 22px; border-radius: var(--radius-sm); border: none;
            font-family: inherit; font-size: 0.9rem; font-weight: 600;
            cursor: pointer; transition: all var(--transition);
            display: inline-flex; align-items: center; gap: 6px;
        }
        .btn-primary { background: var(--primary); color: #fff; }
        .btn-primary:hover { background: var(--primary-dark); transform: translateY(-1px); }
        .btn-success { background: var(--success); color: #fff; }
        .btn-success:hover { background: #059669; transform: translateY(-1px); }
        .btn-warning { background: var(--warning); color: #fff; }
        .btn-warning:hover { background: #d97706; transform: translateY(-1px); }
        .btn-error { background: var(--error); color: #fff; }
        .btn-error:hover { background: #dc2626; transform: translateY(-1px); }
        .btn-outline {
            background: transparent; color: var(--primary);
            border: 2px solid var(--primary);
        }
        .btn-outline:hover { background: var(--primary); color: #fff; }

        /* ─── Forms ─── */
        .form-grid {
            display: grid; grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
            gap: 16px; margin-bottom: 16px;
        }
        .form-group { display: flex; flex-direction: column; }
        .form-group label {
            font-weight: 500; color: var(--text-secondary);
            margin-bottom: 6px; font-size: 0.85rem;
        }
        .form-group input, .form-group select {
            padding: 10px 14px; border: 2px solid var(--border);
            border-radius: var(--radius-sm); font-size: 1rem;
            background: var(--bg-card); color: var(--text);
            font-family: inherit; transition: all var(--transition);
        }
        .form-group input:focus, .form-group select:focus {
            outline: none; border-color: var(--primary);
            box-shadow: 0 0 0 3px rgba(37,99,235,0.15);
        }

        .result-box {
            margin-top: 16px; padding: 20px; border-radius: var(--radius-sm);
            background: linear-gradient(135deg, #ecfdf5, #d1fae5);
            border: 1px solid var(--success);
            opacity: 0; transform: translateY(8px);
            transition: all 0.4s ease;
        }
        [data-theme="dark"] .result-box {
            background: linear-gradient(135deg, #064e3b33, #1e293b);
        }
        .result-box.show { opacity: 1; transform: translateY(0); }
        .working-out {
            background: var(--bg-card); border-radius: 8px; padding: 14px;
            margin-bottom: 12px; border-left: 4px solid var(--info);
        }
        .working-out h5 { color: var(--info); margin-bottom: 8px; font-size: 0.85rem; }
        .working-out .step {
            font-family: 'Courier New', monospace; font-size: 0.85rem;
            color: var(--text-secondary); margin-bottom: 4px;
        }
        .final-answer {
            background: linear-gradient(135deg, var(--primary), var(--accent));
            color: #fff; padding: 10px 16px; border-radius: 8px;
            font-weight: 700; text-align: center; font-size: 1.05rem;
        }

        /* ─── Quiz ─── */
        .quiz-q {
            background: var(--bg); border-radius: var(--radius-sm);
            padding: 20px; margin-bottom: 16px;
            border: 1px solid var(--border); transition: all var(--transition);
        }
        .quiz-q.correct { border-color: var(--success); background: #ecfdf522; }
        .quiz-q.incorrect { border-color: var(--error); background: #fef2f222; }
        .quiz-q p { font-weight: 600; color: var(--text); margin-bottom: 12px; }
        .quiz-option {
            display: flex; align-items: center; gap: 10px;
            padding: 8px 12px; border-radius: 8px; margin-bottom: 6px;
            cursor: pointer; transition: all var(--transition);
        }
        .quiz-option:hover { background: rgba(37,99,235,0.08); }
        .quiz-option input[type="radio"] { width: 18px; height: 18px; accent-color: var(--primary); cursor: pointer; }
        .quiz-option label { cursor: pointer; color: var(--text-secondary); font-size: 0.95rem; flex: 1; }
        .quiz-feedback {
            margin-top: 8px; padding: 10px 14px; border-radius: 8px;
            font-size: 0.9rem; display: none;
        }
        .quiz-feedback.show { display: block; }
        .quiz-feedback.correct-fb { background: #dcfce7; color: #166534; }
        .quiz-feedback.incorrect-fb { background: #fee2e2; color: #991b1b; }
        [data-theme="dark"] .quiz-feedback.correct-fb { background: #064e3b55; color: #6ee7b7; }
        [data-theme="dark"] .quiz-feedback.incorrect-fb { background: #450a0a55; color: #fca5a5; }
        .score-box {
            text-align: center; padding: 24px; border-radius: var(--radius);
            background: linear-gradient(135deg, var(--primary), var(--accent));
            color: #fff; margin-top: 20px;
        }
        .score-box .score-num { font-size: 3rem; font-weight: 800; }
        .score-box .score-text { font-size: 1.1rem; opacity: 0.9; }

        /* ─── Method Steps ─── */
        .method-step {
            display: flex; align-items: flex-start; gap: 14px;
            padding: 12px; margin: 8px 0; border-radius: var(--radius-sm);
            transition: all var(--transition);
        }
        .method-step:hover { background: rgba(37,99,235,0.04); transform: translateX(4px); }
        .step-num {
            width: 32px; height: 32px; border-radius: 50%; flex-shrink: 0;
            background: linear-gradient(135deg, var(--primary), var(--accent));
            color: #fff; display: flex; align-items: center; justify-content: center;
            font-weight: 700; font-size: 0.85rem;
        }
        .step-text { color: var(--text-secondary); padding-top: 4px; }

        .materials-box {
            background: var(--bg); border: 1px solid var(--border);
            border-radius: var(--radius-sm); padding: 16px; margin-bottom: 16px;
        }

        /* ─── Dual Graph Layout ─── */
        .dual-graphs {
            display: grid; grid-template-columns: 1fr 1fr; gap: 16px; margin-top: 12px;
        }
        @media (max-width: 700px) { .dual-graphs { grid-template-columns: 1fr; } }

        .graph-panel {
            background: var(--bg); border: 1px solid var(--border);
            border-radius: var(--radius-sm); padding: 12px; text-align: center;
        }
        .graph-panel h4 { margin-bottom: 8px; font-size: 0.9rem; }
        .graph-panel canvas { width: 100%; height: auto; border-radius: 6px; }

        /* ─── Footer ─── */
        .footer {
            text-align: center; padding: 32px 20px;
            color: var(--text-muted); font-size: 0.85rem;
        }

        /* ─── Responsive ─── */
        @media (max-width: 768px) {
            .card { padding: 20px; }
            .sim-container { padding: 16px; }
            .nav-links { display: none; }
            .form-grid { grid-template-columns: 1fr; }
        }

        /* ─── Animations ─── */
        @keyframes fadeUp {
            from { opacity: 0; transform: translateY(20px); }
            to { opacity: 1; transform: translateY(0); }
        }