/* ─── Variables ─────────────────────────────────────── */
:root {
  --bg:           #ffffff;
  --surface:      #f7f7f7;
  --surface2:     #eeeeee;
  --accent:       #1a1a1a;
  --accent-dim:   rgba(0, 0, 0, 0.04);
  --text:         #1a1a1a;
  --text-muted:   #888888;
  --text-hover:   #444444;
  --border:       #e4e4e4;
  --border-hover: #c0c0c0;
  --nav-bg:       rgba(255, 255, 255, 0.95);

  --font-serif: 'Playfair Display', Georgia, serif;
  --font-sans:  'Inter', system-ui, sans-serif;

  --nav-h: 64px;
}

/* ─── Dark mode (system) ─────────────────────────────── */
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg:           #191A1B;
    --surface:      #212223;
    --surface2:     #2a2a2b;
    --accent:       #e0e0e0;
    --accent-dim:   rgba(255, 255, 255, 0.04);
    --text:         #e0e0e0;
    --text-muted:   #777777;
    --text-hover:   #bbbbbb;
    --border:       #2e2e2f;
    --border-hover: #555555;
    --nav-bg:       rgba(25, 26, 27, 0.92);
  }
}

/* ─── Dark mode (explicit toggle) ────────────────────── */
:root[data-theme="dark"] {
  --bg:           #191A1B;
  --surface:      #212223;
  --surface2:     #2a2a2b;
  --accent:       #e0e0e0;
  --accent-dim:   rgba(255, 255, 255, 0.04);
  --text:         #e0e0e0;
  --text-muted:   #777777;
  --text-hover:   #bbbbbb;
  --border:       #2e2e2f;
  --border-hover: #555555;
  --nav-bg:       rgba(25, 26, 27, 0.92);
}

/* ─── Reset ─────────────────────────────────────────── */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

html { scroll-behavior: smooth; }

body {
  background: var(--bg);
  color: var(--text);
  font-family: var(--font-sans);
  font-weight: 300;
  line-height: 1.7;
  overflow-x: hidden;
}

a { color: inherit; text-decoration: none; }
img { display: block; width: 100%; height: 100%; object-fit: cover; }

/* ─── Navigation ─────────────────────────────────────── */
#nav {
  position: fixed;
  top: 0; left: 0; right: 0;
  z-index: 100;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 48px;
  height: var(--nav-h);
  background: var(--nav-bg);
  border-bottom: 1px solid transparent;
  backdrop-filter: blur(8px);
  -webkit-backdrop-filter: blur(8px);
  transition: border-color 0.3s, box-shadow 0.3s;
}

#nav.scrolled {
  border-bottom-color: var(--border);
  box-shadow: 0 1px 10px rgba(0, 0, 0, 0.06);
}

.nav-name {
  font-family: var(--font-serif);
  font-size: 1.05rem;
  font-weight: 400;
  letter-spacing: 0.03em;
  color: var(--text);
}

.nav-links {
  display: flex;
  gap: 32px;
}

.nav-links a {
  font-size: 0.78rem;
  font-weight: 400;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--text-muted);
  transition: color 0.2s;
}

.nav-links a:hover { color: var(--text); }

/* ─── Sections ───────────────────────────────────────── */
section { padding: 80px 48px; }

.section-header {
  margin-bottom: 48px;
}

.section-title {
  font-family: var(--font-serif);
  font-size: clamp(1.4rem, 3vw, 1.9rem);
  font-weight: 400;
  letter-spacing: 0.01em;
  color: var(--text);
  position: relative;
  display: inline-block;
}

.section-title::after {
  content: '';
  position: absolute;
  left: 0; bottom: -8px;
  width: 28px; height: 2px;
  background: var(--text-muted);
  opacity: 0.35;
}

/* ─── Filter bar ─────────────────────────────────────── */
.filter-bar {
  display: flex;
  gap: 8px;
  margin-bottom: 28px;
  flex-wrap: wrap;
}

.filter-btn {
  padding: 5px 14px;
  font-family: var(--font-sans);
  font-size: 0.78rem;
  font-weight: 500;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  background: transparent;
  border: 1px solid var(--border);
  color: var(--text-muted);
  border-radius: 999px;
  cursor: pointer;
  transition: background 0.2s, color 0.2s, border-color 0.2s;
}

.filter-btn:hover {
  border-color: var(--text-muted);
  color: var(--text);
}

.filter-btn.active {
  background: var(--text);
  border-color: var(--text);
  color: var(--bg);
}

.filter-sep {
  color: var(--text-muted);
  font-size: 0.75rem;
  margin: 0 6px;
  user-select: none;
}

.filter-bar--sub {
  margin-top: -20px;
  margin-bottom: 0;
  max-height: 0;
  overflow: hidden;
  opacity: 0;
  transition: max-height 0.25s ease, opacity 0.2s ease, margin-bottom 0.25s ease;
}
.filter-bar--sub.no-transition {
  transition: none;
}

.filter-bar--sub.open {
  max-height: 120px;
  opacity: 1;
  margin-bottom: 20px;
}

/* ─── Tab bar ────────────────────────────────────────── */
#portfolio { background: var(--bg); }

.tab-bar {
  display: flex;
  gap: 32px;
  margin-bottom: 48px;
  border-bottom: 1px solid var(--border);
}

.tab-btn {
  font-family: var(--font-serif);
  font-size: clamp(1.4rem, 3vw, 1.9rem);
  font-weight: 400;
  color: var(--text-muted);
  background: none;
  border: none;
  padding-bottom: 12px;
  cursor: pointer;
  position: relative;
  transition: color 0.2s;
}

.tab-btn::after {
  content: '';
  position: absolute;
  left: 0; bottom: -1px;
  width: 0; height: 2px;
  background: var(--text);
  transition: width 0.25s ease;
}

.tab-btn.active {
  color: var(--text);
}

.tab-btn.active::after {
  width: 100%;
}

.tab-panel {
  display: none;
}

.tab-panel.active {
  display: block;
}

/* ─── Posts grid (blog cards) ────────────────────────── */

.posts-grid {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: 5px;
  max-width: 1400px;
}

.post-card {
  display: block;
  border: 1px solid var(--border);
  background: var(--bg);
  overflow: hidden;

  opacity: 0;
  transform: translateY(20px);
  transition: opacity 0.5s ease, transform 0.5s ease,
              box-shadow 0.25s, border-color 0.25s;
}

.post-card.visible {
  opacity: 1;
  transform: translateY(0);
}

.post-card:hover {
  box-shadow: 0 6px 32px rgba(0, 0, 0, 0.08);
  border-color: var(--border-hover);
}

.post-thumb {
  aspect-ratio: 4 / 3;
  overflow: hidden;
  background: var(--surface2);
  position: relative;
}

.post-thumb img {
  width: 100%;
  height: 100%;
  object-fit: cover;
  transition: transform 0.45s ease;
}

.post-card:hover .post-thumb img {
  transform: scale(1.04);
}

.post-thumb.placeholder img {
  display: none;
}

.post-thumb.placeholder::after {
  content: attr(data-title);
  position: absolute;
  inset: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 0.75rem;
  color: var(--text-muted);
  letter-spacing: 0.06em;
  padding: 16px;
}

.post-info {
  padding: 20px 24px 26px;
}

.post-category {
  display: block;
  font-size: 0.67rem;
  font-weight: 500;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--text-muted);
  margin-bottom: 8px;
}

.post-title {
  font-family: var(--font-serif);
  font-size: 1.2rem;
  font-weight: 400;
  color: var(--text);
  line-height: 1.3;
  margin-bottom: 10px;
  transition: color 0.2s;
}

.post-card:hover .post-title {
  color: var(--text-hover);
}

.post-date {
  font-size: 0.74rem;
  color: var(--text-muted);
  font-weight: 300;
}

/* ─── Notes grid ─────────────────────────────────────── */
.notes-grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 5px;
}

.note-card {
  display: flex;
  flex-direction: column;
  gap: 10px;
  padding: 20px;
  border: 1px solid var(--border);
  background: var(--bg);
  opacity: 0;
  transform: translateY(16px);
  transition: opacity 0.4s ease, transform 0.4s ease,
              box-shadow 0.25s, border-color 0.25s;
}

.note-card.visible {
  opacity: 1;
  transform: translateY(0);
}

.note-card:hover {
  box-shadow: 0 4px 20px rgba(0, 0, 0, 0.06);
  border-color: var(--border-hover);
}

.note-tags {
  display: flex;
  flex-wrap: wrap;
  gap: 4px;
}

.note-tag {
  font-size: 0.62rem;
  font-weight: 500;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--text-muted);
  background: var(--surface2);
  border: 1px solid var(--border);
  padding: 2px 8px;
  border-radius: 999px;
}

.note-title {
  font-family: var(--font-sans);
  font-size: 0.9rem;
  font-weight: 400;
  color: var(--text);
  line-height: 1.4;
}

.note-date {
  font-size: 0.75rem;
  color: var(--text-muted);
  margin-top: auto;
}

.notes-pager {
  grid-column: 1 / -1;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 20px;
  padding: 32px 0 8px;
}

.notes-pager-btn {
  background: none;
  border: 1px solid var(--border);
  color: var(--text-muted);
  font-size: 0.85rem;
  padding: 6px 14px;
  cursor: pointer;
  transition: color 0.2s, border-color 0.2s;
}
.notes-pager-btn:hover:not(:disabled) {
  color: var(--text);
  border-color: var(--border-hover);
}
.notes-pager-btn:disabled {
  opacity: 0.3;
  cursor: default;
}

.notes-pager-info {
  font-size: 0.78rem;
  color: var(--text-muted);
  letter-spacing: 0.06em;
  min-width: 48px;
  text-align: center;
}

.notes-loading {
  color: var(--text-muted);
  font-size: 0.85rem;
  letter-spacing: 0.08em;
}

/* ─── Contact ────────────────────────────────────────── */
#contact {
  background: var(--surface);
  border-top: 1px solid var(--border);
}

.contact-links {
  display: flex;
  flex-wrap: wrap;
  gap: 16px;
}

.contact-item {
  display: flex;
  align-items: center;
  gap: 12px;
  padding: 14px 24px;
  border: 1px solid var(--border);
  background: var(--bg);
  color: var(--text-muted);
  font-size: 0.88rem;
  letter-spacing: 0.02em;
  transition: color 0.2s, border-color 0.2s, box-shadow 0.2s;
}

.contact-item:hover {
  color: var(--text);
  border-color: var(--border-hover);
  box-shadow: 0 2px 12px rgba(0, 0, 0, 0.06);
}

.contact-icon {
  width: 18px;
  height: 18px;
  flex-shrink: 0;
  opacity: 0.6;
  transition: opacity 0.2s;
}

.contact-item:hover .contact-icon {
  opacity: 1;
}

/* ─── Tag mode toggle ────────────────────────────────── */
#notes-mode-bar {
  display: flex;
  justify-content: flex-end;
  margin-bottom: 20px;
}

.tag-mode-btn {
  font-size: 0.65rem;
  font-weight: 500;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--text-muted);
  background: none;
  border: 1px solid var(--border);
  padding: 4px 12px;
  cursor: pointer;
  transition: color 0.2s, border-color 0.2s, background 0.2s;
}
.tag-mode-btn:hover {
  color: var(--text);
  border-color: var(--border-hover);
}
.tag-mode-btn.active {
  color: var(--bg);
  background: var(--text);
  border-color: var(--text);
}

/* ─── Math filter tag ───────────────────────────────── */
.filter-btn[data-filter="Math"]:hover {
  color: #FF0034;
  border-color: #FF0034;
}
.filter-btn[data-filter="Math"].active {
  background: #FF0034;
  border-color: #FF0034;
  color: #ffffff;
}

/* ─── Python filter tag ─────────────────────────────── */
.filter-btn[data-filter="Python"]:hover {
  color: #336E9F;
  border-color: #336E9F;
}
.filter-btn[data-filter="Python"].active {
  background: #336E9F;
  border-color: #336E9F;
  color: #ffffff;
}

/* ─── Houdini filter tag ─────────────────────────────── */
.filter-btn[data-filter="Houdini"]:hover {
  color: #FF6600;
  border-color: #FF6600;
}
.filter-btn[data-filter="Houdini"].active {
  background: #FF6600;
  border-color: #FF6600;
  color: #ffffff;
}

/* ─── Theme toggle switch ────────────────────────────── */
#theme-toggle {
  display: flex;
  align-items: center;
  background: none;
  border: none;
  cursor: pointer;
  padding: 0;
}

.toggle-track {
  position: relative;
  width: 34px;
  height: 18px;
  border-radius: 999px;
  border: 1px solid var(--border);
  background: var(--surface2);
  transition: background 0.25s, border-color 0.25s;
}

.toggle-thumb {
  position: absolute;
  top: 2px;
  left: 2px;
  width: 12px;
  height: 12px;
  border-radius: 50%;
  background: var(--text-muted);
  transition: transform 0.25s, background 0.25s;
}

/* Dark state: thumb slides right */
[data-theme="dark"] .toggle-thumb {
  transform: translateX(16px);
  background: var(--text);
}
[data-theme="dark"] .toggle-track {
  border-color: var(--border-hover);
}

@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) .toggle-thumb {
    transform: translateX(16px);
    background: var(--text);
  }
  :root:not([data-theme="light"]) .toggle-track {
    border-color: var(--border-hover);
  }
}

/* ─── Footer ─────────────────────────────────────────── */
footer {
  padding: 28px 48px;
  background: var(--bg);
  border-top: 1px solid var(--border);
  text-align: center;
  font-size: 0.74rem;
  letter-spacing: 0.06em;
  color: var(--text-muted);
}

/* ─── Responsive ─────────────────────────────────────── */
@media (max-width: 960px) {
  .posts-grid { grid-template-columns: repeat(2, 1fr); gap: 5px; }
}

@media (max-width: 640px) {
  #nav { padding: 0 20px; }
  #intro { padding: calc(var(--nav-h) + 48px) 20px 52px; }
  section { padding: 60px 20px; }
  footer { padding: 24px 20px; }
  .posts-grid { grid-template-columns: 1fr; gap: 16px; }
  .notes-grid { grid-template-columns: 1fr; }
  .nav-links { gap: 20px; }
  .contact-links { flex-direction: column; }
  .contact-item { width: 100%; }
}