/* ===== Lian Zeng – Light theme (mechanical engineering) ===== */
:root {
  --font-sans: "DM Sans", system-ui, sans-serif;
  --font-display: "Outfit", system-ui, sans-serif;
  --color-bg: #f8f9fa;
  --color-surface: #ffffff;
  --color-surface-2: #f0f2f5;
  --color-text: #1a1d21;
  --color-text-muted: #5c6269;
  --color-accent: #155bd5;
  --color-accent-hover: #1249a8;
  --color-border: #e2e5e9;
  --color-success: #0d7d3d;
  --color-error: #c41e3a;
  --radius: 8px;
  --radius-lg: 12px;
  --shadow: 0 2px 12px rgba(0, 0, 0, 0.08);
  --shadow-strong: 0 4px 20px rgba(0, 0, 0, 0.12);
  --transition: 0.2s ease;
}

*,
*::before,
*::after {
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
}

body {
  margin: 0;
  font-family: var(--font-sans);
  font-size: 1rem;
  line-height: 1.6;
  color: var(--color-text);
  background: var(--color-bg);
}

img {
  max-width: 100%;
  height: auto;
  display: block;
}

a {
  color: var(--color-accent);
  text-decoration: none;
  transition: color var(--transition);
}
a:hover {
  color: var(--color-accent-hover);
}

.container {
  width: 100%;
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 1.25rem;
}

.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border: 0;
}

/* ----- Header ----- */
.site-header {
  position: sticky;
  top: 0;
  z-index: 100;
  background: rgba(255, 255, 255, 0.95);
  backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--color-border);
  box-shadow: 0 1px 0 var(--color-border);
}

.nav {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  min-height: 4rem;
}

.nav-brand {
  font-family: var(--font-display);
  font-weight: 700;
  font-size: 1.25rem;
  color: var(--color-text);
}
.nav-brand:hover {
  color: var(--color-text);
}

.nav-menu {
  display: flex;
  align-items: center;
  gap: 0.25rem;
  margin: 0;
  padding: 0;
  list-style: none;
}

.nav-link {
  padding: 0.5rem 0.85rem;
  color: var(--color-text-muted);
  font-weight: 500;
  border-radius: var(--radius);
}
.nav-link:hover {
  color: var(--color-text);
  background: var(--color-surface-2);
}
.nav-link.active {
  color: var(--color-accent);
  background: rgba(21, 91, 213, 0.08);
}

.nav-toggle {
  display: none;
  flex-direction: column;
  justify-content: center;
  gap: 5px;
  width: 40px;
  height: 40px;
  padding: 0;
  background: transparent;
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  cursor: pointer;
  color: var(--color-text);
}
.nav-toggle span {
  display: block;
  height: 2px;
  background: currentColor;
  border-radius: 1px;
}

@media (max-width: 768px) {
  .nav-toggle {
    display: flex;
  }
  .nav-menu {
    position: absolute;
    top: 100%;
    left: 0;
    right: 0;
    flex-direction: column;
    padding: 1rem;
    background: var(--color-surface);
    border-bottom: 1px solid var(--color-border);
    box-shadow: var(--shadow-strong);
    display: none;
  }
  .nav-menu.is-open {
    display: flex;
  }
}

/* ----- Hero ----- */
.hero {
  padding: 4rem 0;
  text-align: center;
}

.hero-inner {
  max-width: 720px;
  margin: 0 auto;
}

.hero-logo {
  margin: 0 auto 1.5rem;
  max-height: 120px;
  width: auto;
}

.hero-tagline {
  margin: 0;
  font-size: 1.125rem;
  color: var(--color-text-muted);
  line-height: 1.7;
}

/* ----- Section title ----- */
.section-title {
  font-family: var(--font-display);
  font-size: 1.75rem;
  font-weight: 700;
  margin: 0 0 1rem;
  color: var(--color-text);
}
.section-title--center {
  text-align: center;
  margin-bottom: 2rem;
}

/* ----- About ----- */
.about {
  padding: 4rem 0;
  background: var(--color-surface);
  border-top: 1px solid var(--color-border);
}

.about-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 3rem;
  align-items: center;
}

.about-media img {
  width: 100%;
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow);
}

.about-content p {
  margin: 0;
  color: var(--color-text-muted);
}

@media (max-width: 768px) {
  .about-grid {
    grid-template-columns: 1fr;
  }
  .about-media {
    order: -1;
  }
}

/* ----- Portfolio ----- */
.portfolio {
  padding: 4rem 0;
  background: var(--color-bg);
}

.portfolio-filters {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 0.5rem;
  margin-bottom: 2rem;
}

.filter-btn {
  padding: 0.5rem 1rem;
  font-family: var(--font-sans);
  font-size: 0.9rem;
  font-weight: 500;
  color: var(--color-text-muted);
  background: var(--color-surface-2);
  border: 1px solid var(--color-border);
  border-radius: 999px;
  cursor: pointer;
  transition: color var(--transition), background var(--transition), border-color var(--transition);
}
.filter-btn:hover,
.filter-btn.active {
  color: #fff;
  background: var(--color-accent);
  border-color: var(--color-accent);
}

.portfolio-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(320px, 1fr));
  gap: 1.5rem;
}

.portfolio-card {
  background: var(--color-surface);
  border-radius: var(--radius-lg);
  overflow: hidden;
  border: 1px solid var(--color-border);
  transition: transform var(--transition), box-shadow var(--transition);
}
.portfolio-card:hover {
  transform: translateY(-4px);
  box-shadow: var(--shadow-strong);
  border-color: var(--color-border);
}
.portfolio-card.is-hidden {
  display: none;
}

.portfolio-card-image {
  position: relative;
  aspect-ratio: 4/3;
  overflow: hidden;
}
.portfolio-card-image img {
  width: 100%;
  height: 100%;
  object-fit: cover;
}
.portfolio-zoom {
  position: absolute;
  inset: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  background: rgba(0, 0, 0, 0.5);
  color: #fff;
  font-weight: 600;
  opacity: 0;
  transition: opacity var(--transition);
}
.portfolio-card-image:hover .portfolio-zoom {
  opacity: 1;
}

.portfolio-card-body {
  padding: 1.25rem;
}

.portfolio-card-title {
  font-family: var(--font-display);
  font-size: 1.1rem;
  font-weight: 600;
  margin: 0 0 0.5rem;
  color: var(--color-text);
}
.portfolio-card-body p {
  margin: 0;
  font-size: 0.9rem;
  color: var(--color-text-muted);
  line-height: 1.5;
}

/* ----- Contact ----- */
.contact {
  padding: 4rem 0;
  background: var(--color-surface);
  border-top: 1px solid var(--color-border);
}

.contact-layout {
  display: grid;
  grid-template-columns: 340px 1fr;
  gap: 3rem;
  align-items: start;
}

.contact-info {
  display: flex;
  flex-direction: column;
  gap: 1.5rem;
}

.contact-info-item {
  display: flex;
  gap: 1rem;
  align-items: flex-start;
}
.contact-icon {
  font-size: 1.25rem;
  flex-shrink: 0;
}
.contact-info-item h4 {
  margin: 0 0 0.25rem;
  font-size: 0.9rem;
  font-weight: 600;
  color: var(--color-text);
}
.contact-info-item p {
  margin: 0;
  font-size: 0.95rem;
  color: var(--color-text-muted);
}
.contact-info-item a {
  color: var(--color-text-muted);
}
.contact-info-item a:hover {
  color: var(--color-accent);
}

.contact-form-wrap {
  max-width: 560px;
}

.contact-form {
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

.form-row {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 1rem;
}

.contact-form input,
.contact-form textarea {
  width: 100%;
  padding: 0.85rem 1rem;
  font-family: var(--font-sans);
  font-size: 1rem;
  color: var(--color-text);
  background: var(--color-surface-2);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  transition: border-color var(--transition);
}
.contact-form input::placeholder,
.contact-form textarea::placeholder {
  color: var(--color-text-muted);
}
.contact-form input:focus,
.contact-form textarea:focus {
  outline: none;
  border-color: var(--color-accent);
}
.contact-form textarea {
  resize: vertical;
  min-height: 120px;
}

.form-actions {
  margin-top: 0.5rem;
}

.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  padding: 0.85rem 1.75rem;
  font-family: var(--font-sans);
  font-size: 1rem;
  font-weight: 600;
  border: none;
  border-radius: var(--radius);
  cursor: pointer;
  transition: background var(--transition), transform var(--transition);
}
.btn:active {
  transform: scale(0.98);
}
.btn-primary {
  color: #fff;
  background: var(--color-accent);
}
.btn-primary:hover {
  background: var(--color-accent-hover);
}

@media (max-width: 768px) {
  .contact-layout {
    grid-template-columns: 1fr;
  }
  .form-row {
    grid-template-columns: 1fr;
  }
}

/* ----- Map ----- */
.map-section {
  width: 100%;
  line-height: 0;
}
.map-section iframe {
  display: block;
}

/* ----- Footer ----- */
.site-footer {
  padding: 2rem 0;
  border-top: 1px solid var(--color-border);
  text-align: center;
}
.footer-copy {
  margin: 0;
  font-size: 0.9rem;
  color: var(--color-text-muted);
}

/* ----- Scroll top ----- */
.scroll-top {
  position: fixed;
  bottom: 1.5rem;
  right: 1.5rem;
  width: 48px;
  height: 48px;
  display: none;
  align-items: center;
  justify-content: center;
  font-size: 1.25rem;
  color: var(--color-text);
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: 50%;
  box-shadow: var(--shadow-strong);
  cursor: pointer;
  z-index: 50;
  transition: background var(--transition), color var(--transition);
}
.scroll-top:hover {
  background: var(--color-accent);
  color: #fff;
  border-color: var(--color-accent);
}
.scroll-top.is-visible {
  display: flex;
}

/* ----- Lightbox ----- */
.lightbox {
  position: fixed;
  inset: 0;
  z-index: 200;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 2rem;
  background: rgba(0, 0, 0, 0.9);
  opacity: 0;
  visibility: hidden;
  transition: opacity var(--transition), visibility var(--transition);
}
.lightbox.is-open {
  opacity: 1;
  visibility: visible;
}
.lightbox img {
  max-width: 100%;
  max-height: 90vh;
  object-fit: contain;
}
.lightbox-close {
  position: absolute;
  top: 1rem;
  right: 1rem;
  width: 48px;
  height: 48px;
  font-size: 2rem;
  line-height: 1;
  color: #fff;
  background: transparent;
  border: none;
  cursor: pointer;
  opacity: 0.8;
}
.lightbox-close:hover {
  opacity: 1;
}

/* ----- Flash messages (after form submit) ----- */
.alert {
  padding: 1rem 1.25rem;
  border-radius: var(--radius);
  margin-bottom: 1rem;
}
.alert-success {
  background: rgba(13, 125, 61, 0.1);
  color: var(--color-success);
  border: 1px solid var(--color-success);
}
.alert-error {
  background: rgba(196, 30, 58, 0.1);
  color: var(--color-error);
  border: 1px solid var(--color-error);
}