/* ── reset & base ── */
        *,
        *::before,
        *::after {
            box-sizing: border-box;
            margin: 0;
            padding: 0;
        }
        html {
            scroll-behavior: smooth;
        }
        body {
            font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
            background: #f7f9fc;
            color: #1a2634;
            line-height: 1.7;
            padding: 0;
            margin: 0;
        }
        a {
            color: #0a6b8a;
            text-decoration: none;
        }
        a:hover {
            text-decoration: underline;
        }
        img {
            max-width: 100%;
            height: auto;
            display: block;
        }
        .container {
            max-width: 1200px;
            margin: 0 auto;
            padding: 0 24px;
        }

        /* ── header ── */
        .site-header {
            background: #0b1a2a;
            color: #fff;
            padding: 18px 0;
            position: sticky;
            top: 0;
            z-index: 100;
            box-shadow: 0 2px 12px rgba(0, 0, 0, 0.15);
        }
        .site-header .container {
            display: flex;
            align-items: center;
            justify-content: space-between;
            flex-wrap: wrap;
            gap: 12px;
        }
        .logo {
            font-weight: 800;
            font-size: 1.25rem;
            letter-spacing: -0.5px;
            display: flex;
            align-items: center;
            gap: 8px;
        }
        .logo span {
            color: #5fc3e4;
        }
        .logo small {
            font-weight: 400;
            font-size: 0.7rem;
            opacity: 0.7;
            margin-left: 6px;
        }
           /* ── navigation ── */
        .btn {
            display: inline-block;
            padding: 7px 18px;
            border-radius: 21px;
            font-size: .5rem;
	    font-weight: 500;
            cursor: pointer;
            transition: var(--transition);
            border: none;  
            text-align: center;
            background: #1e88b3;
            color: #fff;
            text-shadow: 2px 2px 2px #000;
            border: 2px solid #FFF;
        }
        .btn:hover {
            background: #2a9fd6;
            text-decoration: none;
            transform: translateY(-2px);
            box-shadow: 0 8px 20px rgba(30, 136, 179, 0.3);
        }
        .nav-links {
            display: flex;
            gap: 8px;
            font-size: 0.9rem;
            font-weight: 500;
            flex-wrap: wrap;
            align-items: center;
        }
        .hero-buttons {
            display: flex;
            flex-wrap: wrap;
            gap: 8px;
            
        }
        .hero-buttons .btn {
            padding: 4px 10px;
            font-size: .8rem;
            
        }
      .hero-buttons .btn-outline {
	    border: 2px solid #1e88b3;
            border-color: rgba(255, 255, 255, 0.5)
            color: #fff;
        }
        .hero-buttons .btn-outline:hover {
            background: #cc7a00;            
            border-color: #fff;
        }
      .nav-links>a,
        .nav-links .dropbtn {
            color: #d0e4f0;
            transition: color 0.2s;
            padding: 6px 12px;
            border-radius: 6px; 
            display: inline-flex;
            align-items: center;
            gap: 4px;
            cursor: pointer;
            background: none;
            border: none;
            font-family: inherit;
            font-size: 0.8rem;
            font-weight: 600;
        }


        /* ── hero ── */
        .hero {
            background: linear-gradient(145deg, #0b1a2a 0%, #1a3450 100%);
            color: #fff;
            padding: 10px 0 30px;
            border-bottom: 4px solid #2a9fd6;
        }
        .hero h1 {
            font-size: 2.8rem;
            font-weight: 800;
            letter-spacing: -1px;
            line-height: 1.2;
            margin-bottom: 16px;
        }
        .hero h1 span {
            color: #5fc3e4;
        }
        .hero p {
            font-size: 1.0rem;
            opacity: 0.9;
            max-width: 720px;
            margin-bottom: 24px;
        }
        .hero-meta {
            display: flex;
            flex-wrap: wrap;
            gap: 20px 36px;
            font-size: 0.9rem;
            opacity: 0.8;
            border-top: 1px solid rgba(255, 255, 255, 0.1);
            padding-top: 20px;
            margin-top: 8px;
        }
        .hero-meta span {
            display: flex;
            align-items: center;
            gap: 6px;
        }

        /* ── toc ── */
        .toc-wrap {
            background: #fff;
            border-radius: 12px;
            padding: 24px 28px;
            margin: 32px 0 40px;
            box-shadow: 0 2px 8px rgba(0, 0, 0, 0.06);
            border: 1px solid #e8edf3;
        }
        .toc-wrap h3 {
            font-size: 1rem;
            text-transform: uppercase;
            letter-spacing: 0.5px;
            color: #4a5a6a;
            margin-bottom: 12px;
        }
        .toc-grid {
            display: grid;
            grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
            gap: 4px 24px;
            font-size: 0.9rem;
        }
        .toc-grid a {
            color: #1a3450;
            padding: 4px 0;
            border-bottom: 1px solid transparent;
            transition: border-color 0.2s;
        }
        .toc-grid a:hover {
            border-color: #2a9fd6;
            text-decoration: none;
        }

        /* ── article layout ── */
        .article-wrapper {
            display: grid;
            grid-template-columns: 1fr 300px;
            gap: 40px;
            margin: 32px 0 60px;
        }
        .main-content {
            background: #fff;
            border-radius: 16px;
            padding: 40px 44px;
            box-shadow: 0 2px 12px rgba(0, 0, 0, 0.05);
            border: 1px solid #e8edf3;
        }
        .sidebar {
            position: sticky;
            top: 100px;
            align-self: start;
        }

        /* ── article typography ── */
        .main-content h2 {
            font-size: 1.8rem;
            font-weight: 700;
            margin: 48px 0 16px;
            padding-top: 8px;
            border-bottom: 2px solid #eef3f8;
            padding-bottom: 10px;
            color: #0b1a2a;
        }
        .main-content h2:first-of-type {
            margin-top: 0;
        }
        .main-content h3 {
            font-size: 1.25rem;
            font-weight: 600;
            margin: 28px 0 12px;
            color: #1a3450;
        }
        .main-content h4 {
            font-size: 1.05rem;
            font-weight: 600;
            margin: 20px 0 8px;
            color: #2a4a6a;
        }
        .main-content p {
            margin-bottom: 16px;
        }
        .main-content ul,
        .main-content ol {
            margin: 12px 0 20px 24px;
        }
        .main-content li {
            margin-bottom: 6px;
        }
        .main-content blockquote {
            border-left: 4px solid #2a9fd6;
            padding: 12px 20px;
            margin: 20px 0;
            background: #f0f6fc;
            border-radius: 0 8px 8px 0;
            font-style: italic;
            color: #1a3450;
        }
        .main-content .note-box {
            background: #e8f4fa;
            border-radius: 8px;
            padding: 16px 20px;
            margin: 20px 0;
            border-left: 4px solid #1e88b3;
        }
        .main-content .warning-box {
            background: #fff3e8;
            border-radius: 8px;
            padding: 16px 20px;
            margin: 20px 0;
            border-left: 4px solid #d97a2a;
        }

        /* ── tables ── */
        .table-wrap {
            overflow-x: auto;
            margin: 24px 0;
        }
        .table-wrap table {
            width: 100%;
            border-collapse: collapse;
            font-size: 0.9rem;
            background: #fff;
            border-radius: 8px;
            overflow: hidden;
        }
        .table-wrap th {
            background: #0b1a2a;
            color: #fff;
            font-weight: 600;
            padding: 10px 14px;
            text-align: left;
        }
        .table-wrap td {
            padding: 10px 14px;
            border-bottom: 1px solid #e8edf3;
        }
        .table-wrap tr:nth-child(even) td {
            background: #f8fafc;
        }
        .table-wrap tr:hover td {
            background: #eef3f8;
        }

        /* ── calculator ── */
        .calc-card {
            background: #f0f6fc;
            border-radius: 12px;
            padding: 24px 28px;
            margin: 32px 0;
            border: 1px solid #d5e3ee;
        }
        .calc-card h3 {
            margin-top: 0;
            color: #0b1a2a;
            font-size: 1.3rem;
            display: flex;
            align-items: center;
            gap: 10px;
        }
        .calc-grid {
            display: grid;
            grid-template-columns: 1fr 1fr;
            gap: 16px 24px;
            margin: 16px 0 20px;
        }
        .calc-group label {
            display: block;
            font-weight: 600;
            font-size: 0.85rem;
            color: #1a3450;
            margin-bottom: 4px;
        }
        .calc-group input,
        .calc-group select {
            width: 100%;
            padding: 10px 12px;
            border: 1px solid #cbd8e5;
            border-radius: 8px;
            font-size: 1rem;
            font-family: inherit;
            background: #fff;
            transition: border 0.2s;
        }
        .calc-group input:focus,
        .calc-group select:focus {
            outline: none;
            border-color: #2a9fd6;
            box-shadow: 0 0 0 3px rgba(42, 159, 214, 0.2);
        }
        .calc-results {
            background: #fff;
            border-radius: 8px;
            padding: 16px 20px;
            margin-top: 16px;
            display: grid;
            grid-template-columns: 1fr 1fr;
            gap: 8px 24px;
            border: 1px solid #d5e3ee;
        }
        .calc-results .result-item {
            display: flex;
            justify-content: space-between;
            padding: 4px 0;
            border-bottom: 1px dashed #e8edf3;
        }
        .calc-results .result-item .label {
            font-weight: 500;
            color: #4a5a6a;
        }
        .calc-results .result-item .value {
            font-weight: 700;
            color: #0b1a2a;
            font-family: 'Inter', monospace;
        }
        .calc-btn {
            background: #1e88b3;
            color: #fff;
            border: none;
            padding: 12px 28px;
            border-radius: 30px;
            font-weight: 600;
            font-size: 1rem;
            cursor: pointer;
            transition: background 0.2s;
            margin-top: 8px;
        }
        .calc-btn:hover {
            background: #2a9fd6;
        }
        .calc-btn.secondary {
            background: #4a5a6a;
        }
        .calc-btn.secondary:hover {
            background: #5a6a7a;
        }

        /* ── sidebar widgets ── */
        .sidebar-widget {
            background: #fff;
            border-radius: 12px;
            padding: 20px 22px;
            margin-bottom: 24px;
            border: 1px solid #e8edf3;
            box-shadow: 0 2px 8px rgba(0, 0, 0, 0.04);
        }
        .sidebar-widget h4 {
            font-size: 1rem;
            font-weight: 700;
            color: #0b1a2a;
            margin-bottom: 12px;
            border-bottom: 2px solid #eef3f8;
            padding-bottom: 8px;
        }
        .sidebar-widget ul {
            list-style: none;
            padding: 0;
            margin: 0;
        }
        .sidebar-widget ul li {
            padding: 6px 0;
            border-bottom: 1px solid #f0f4f8;
        }
        .sidebar-widget ul li:last-child {
            border-bottom: none;
        }
        .sidebar-widget ul li a {
            color: #1a3450;
            font-weight: 500;
        }
        .sidebar-widget ul li a:hover {
            color: #1e88b3;
            text-decoration: none;
        }
        .sidebar-widget .tag-cloud {
            display: flex;
            flex-wrap: wrap;
            gap: 6px;
        }
        .sidebar-widget .tag-cloud span {
            background: #eef3f8;
            padding: 4px 12px;
            border-radius: 20px;
            font-size: 0.75rem;
            font-weight: 500;
            color: #1a3450;
        }

        /* ── FAQ accordion ── */
        .faq-item {
            border: 1px solid #e8edf3;
            border-radius: 8px;
            margin-bottom: 10px;
            overflow: hidden;
        }
        .faq-item summary {
            padding: 16px 20px;
            font-weight: 600;
            cursor: pointer;
            background: #f8fafc;
            transition: background 0.2s;
            list-style: none;
            display: flex;
            justify-content: space-between;
            align-items: center;
        }
        .faq-item summary::-webkit-details-marker {
            display: none;
        }
        .faq-item summary::after {
            content: "+";
            font-size: 1.3rem;
            font-weight: 300;
            color: #2a9fd6;
        }
        .faq-item[open] summary::after {
            content: "−";
        }
        .faq-item summary:hover {
            background: #eef3f8;
        }
        .faq-item .faq-answer {
            padding: 0 20px 20px;
            color: #2a3a4a;
        }

        /* ── footer ── */
        .site-footer {
            background: #0b1a2a;
            color: #b0c8d8;
            padding: 40px 0 28px;
            border-top: 4px solid #1e88b3;
            margin-top: 20px;
        }
        .site-footer .container {
            display: grid;
            grid-template-columns: 2fr 1fr 1fr;
            gap: 32px;
        }
        .site-footer h5 {
            color: #fff;
            font-size: 1rem;
            margin-bottom: 12px;
        }
        .site-footer a {
            color: #b0c8d8;
        }
        .site-footer a:hover {
            color: #fff;
            text-decoration: none;
        }
        .site-footer ul {
            list-style: none;
            padding: 0;
        }
        .site-footer ul li {
            padding: 4px 0;
        }
        .footer-bottom {
            grid-column: 1 / -1;
            border-top: 1px solid rgba(255, 255, 255, 0.08);
            padding-top: 20px;
            margin-top: 12px;
            font-size: 0.85rem;
            text-align: center;
        }

        /* ── responsive ── */
        @media (max-width: 1024px) {
            .article-wrapper {
                grid-template-columns: 1fr;
            }
            .sidebar {
                position: static;
            }
        }
        @media (max-width: 768px) {
            .hero h1 {
                font-size: 2rem;
            }
            .main-content {
                padding: 24px 18px;
            }
            .calc-grid {
                grid-template-columns: 1fr;
            }
            .calc-results {
                grid-template-columns: 1fr;
            }
            .site-footer .container {
                grid-template-columns: 1fr;
                gap: 24px;
            }
            .toc-grid {
                grid-template-columns: 1fr 1fr;
            }
            .site-header .container {
                flex-direction: column;
                align-items: stretch;
            }
            .nav-links {
                justify-content: center;
            }
        }
        @media (max-width: 480px) {
            .toc-grid {
                grid-template-columns: 1fr;
            }
            .hero-meta {
                flex-direction: column;
                gap: 6px;
            }
        }

        /* ── utility ── */
        .text-center {
            text-align: center;
        }
        .mt-8 {
            margin-top: 8px;
        }
        .mt-16 {
            margin-top: 16px;
        }
        .mt-24 {
            margin-top: 24px;
        }
        .gap-8 {
            gap: 8px;
        }
        .flex-wrap {
            display: flex;
            flex-wrap: wrap;
            gap: 12px;
            align-items: center;
        }
        .badge {
            display: inline-block;
            background: #1e88b3;
            color: #fff;
            font-size: 0.7rem;
            font-weight: 600;
            padding: 2px 12px;
            border-radius: 20px;
            letter-spacing: 0.3px;
        }
        .badge.gold {
            background: #c98f2a;
        }