:root {
  --primary: #2563eb;
  --primary-hover: #1d4ed8;
  --bg: #f8fafc;
  --surface: #ffffff;
  --text-main: #0f172a;
  --text-muted: #64748b;
  --border: #e2e8f0;
  --shadow: 0 4px 6px -1px rgb(0 0 0 / 0.07), 0 2px 4px -2px rgb(0 0 0 / 0.05);
  --radius: 12px;
}

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen, Ubuntu, Cantarell, sans-serif;
}

body {
  background-color: var(--bg);
  color: var(--text-main);
  line-height: 1.5;
  padding-bottom: 60px;
}

header {
  background: var(--surface);
  border-bottom: 1px solid var(--border);
  padding: 20px 0;
  text-align: center;
  margin-bottom: 40px;
}

header h1 {
  font-size: 1.8rem;
  font-weight: 800;
  color: var(--primary);
}

header p {
  color: var(--text-muted);
  font-size: 0.95rem;
  margin-top: 4px;
}

.container {
  max-width: 1100px;
  margin: 0 auto;
  padding: 0 20px;
}

/* Category Grid */
.category-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(320px, 1fr));
  gap: 24px;
}

.card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 24px;
  box-shadow: var(--shadow);
  display: flex;
  flex-direction: column;
}

.card-header {
  margin-bottom: 16px;
  padding-bottom: 12px;
  border-bottom: 1px solid var(--border);
}

.card-header h2 {
  font-size: 1.3rem;
  color: var(--text-main);
}

.card-header span {
  font-size: 0.8rem;
  color: var(--primary);
  font-weight: 600;
  text-transform: uppercase;
}

.tool-list {
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.tool-list a {
  text-decoration: none;
  color: var(--text-muted);
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 8px 10px;
  border-radius: 6px;
  transition: all 0.2s ease;
}

.tool-list a:hover {
  background: #eff6ff;
  color: var(--primary);
}

/* Calculator View Layout */
.calculator-box {
  max-width: 550px;
  margin: 0 auto;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 32px;
  box-shadow: var(--shadow);
}

.form-group {
  margin-bottom: 18px;
}

.form-group label {
  display: block;
  font-weight: 600;
  margin-bottom: 6px;
  font-size: 0.9rem;
}

.form-group input, .form-group select {
  width: 100%;
  padding: 10px 14px;
  border: 1px solid var(--border);
  border-radius: 8px;
  font-size: 1rem;
}

.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);
}

.btn-calc {
  width: 100%;
  padding: 12px;
  background: var(--primary);
  color: #fff;
  border: none;
  border-radius: 8px;
  font-size: 1rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.2s;
}

.btn-calc:hover {
  background: var(--primary-hover);
}

.result-box {
  margin-top: 24px;
  padding: 16px;
  background: #f1f5f9;
  border-radius: 8px;
  display: none;
}

.result-row {
  display: flex;
  justify-content: space-between;
  margin-bottom: 8px;
}

.back-link {
  display: inline-block;
  margin-bottom: 20px;
  text-decoration: none;
  color: var(--primary);
  font-weight: 600;
}