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

body {
    font-family: 'Segoe UI', system-ui, sans-serif;
    background: #f5f7fa;
    color: #1e4b69;
    min-height: 100vh;
}

.container {
    max-width: 900px;
    margin: 0 auto;
    padding: 24px 20px;
}

h1 {
    text-align: center;
    color: #1e4b69;
    font-size: 1.5rem;
    margin-bottom: 4px;
}

.subtitle {
    text-align: center;
    color: #6b7280;
    font-size: 0.88rem;
    margin-bottom: 28px;
}

.timeline {
    position: relative;
    display: flex;
    flex-direction: column;
    gap: 16px;
}

.timeline-track {
    position: absolute;
    left: 22px;
    top: 0;
    bottom: 0;
    width: 3px;
    background: linear-gradient(to bottom, #3b82f6, #22c55e, #f59e0b, #7c3aed, #ef4444);
    border-radius: 2px;
}

.stage-card {
    margin-left: 50px;
    background: #fff;
    border-radius: 12px;
    padding: 16px;
    cursor: pointer;
    transition: box-shadow 0.3s, transform 0.2s;
    border: 2px solid #e5e7eb;
    position: relative;
}

.stage-card::before {
    content: '';
    position: absolute;
    left: -36px;
    top: 20px;
    width: 14px;
    height: 14px;
    border-radius: 50%;
    background: #fff;
    border: 3px solid var(--stage-color, #1e4b69);
    z-index: 1;
}

.stage-card:hover {
    box-shadow: 0 4px 16px rgba(30, 75, 105, 0.12);
    transform: translateX(4px);
}

.stage-card.active {
    border-color: var(--active-color, #1e4b69);
    box-shadow: 0 4px 20px rgba(30, 75, 105, 0.15);
}

.stage-card[data-stage="product-sense"] { --active-color: #3b82f6; }
.stage-card[data-stage="product-sense"]::before { border-color: #3b82f6; }
.stage-card[data-stage="system-design"] { --active-color: #22c55e; }
.stage-card[data-stage="system-design"]::before { border-color: #22c55e; }
.stage-card[data-stage="technical-deep-dive"] { --active-color: #f59e0b; }
.stage-card[data-stage="technical-deep-dive"]::before { border-color: #f59e0b; }
.stage-card[data-stage="analytical"] { --active-color: #7c3aed; }
.stage-card[data-stage="analytical"]::before { border-color: #7c3aed; }
.stage-card[data-stage="behavioral"] { --active-color: #ef4444; }
.stage-card[data-stage="behavioral"]::before { border-color: #ef4444; }

.stage-header {
    display: flex;
    align-items: center;
    gap: 12px;
    margin-bottom: 8px;
}

.stage-number {
    width: 32px;
    height: 32px;
    border-radius: 8px;
    display: flex;
    align-items: center;
    justify-content: center;
    font-weight: 700;
    font-size: 1rem;
    color: #fff;
    background: var(--stage-color);
    flex-shrink: 0;
}

.stage-info {
    flex: 1;
    display: flex;
    align-items: center;
    justify-content: space-between;
}

.stage-info h3 {
    font-size: 1rem;
    color: #1e4b69;
}

.duration {
    font-size: 0.75rem;
    font-weight: 600;
    padding: 3px 10px;
    border-radius: 12px;
    background: #f0f4f8;
    color: #6b7280;
}

.stage-desc {
    font-size: 0.88rem;
    color: #4b5563;
    margin-bottom: 8px;
    line-height: 1.4;
}

.pitfall {
    font-size: 0.82rem;
    color: #dc2626;
    background: #fef2f2;
    padding: 8px 12px;
    border-radius: 6px;
    border-left: 3px solid #ef4444;
    cursor: help;
    transition: background 0.2s;
}

.pitfall:hover {
    background: #fee2e2;
}

.stage-detail {
    display: none;
    margin-top: 14px;
    padding-top: 14px;
    border-top: 1px solid #e5e7eb;
    animation: slideDown 0.3s ease;
}

.stage-detail.visible {
    display: block;
}

@keyframes slideDown {
    from { opacity: 0; max-height: 0; }
    to { opacity: 1; max-height: 500px; }
}

.stage-detail h4 {
    font-size: 0.88rem;
    color: #1e4b69;
    margin-bottom: 8px;
    margin-top: 12px;
}

.stage-detail h4:first-child {
    margin-top: 0;
}

.stage-detail ul {
    list-style: none;
    display: flex;
    flex-direction: column;
    gap: 4px;
    padding-left: 0;
}

.stage-detail li {
    font-size: 0.84rem;
    color: #4b5563;
    padding: 6px 10px;
    background: #f5f7fa;
    border-radius: 6px;
    line-height: 1.4;
}

.stage-detail li::before {
    content: '\2713';
    margin-right: 8px;
    font-weight: 700;
    color: #22c55e;
}

.tooltip {
    display: none;
    position: fixed;
    background: #1e293b;
    color: #fff;
    padding: 10px 14px;
    border-radius: 8px;
    font-size: 0.82rem;
    line-height: 1.5;
    max-width: 300px;
    z-index: 1000;
    pointer-events: none;
    box-shadow: 0 4px 16px rgba(0, 0, 0, 0.2);
}

@media (max-width: 600px) {
    .timeline-track {
        left: 14px;
    }

    .stage-card {
        margin-left: 36px;
        padding: 12px;
    }

    .stage-card::before {
        left: -28px;
        width: 10px;
        height: 10px;
    }

    .stage-info {
        flex-direction: column;
        align-items: flex-start;
        gap: 4px;
    }
}