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

:root {
  --color-bg: #fff;
  --color-text: #333;
  --color-text-muted: #595959;
  --color-text-light: #999;
  --color-accent: #007acc;
  --color-border: #e9ecef;
  --color-bg-hover: #f5f5f5;
  --font-sans: "Helvetica Neue", Helvetica, Arial, "Segoe UI", sans-serif;
  --spacing-xs: 0.5rem;
  --spacing-sm: 1rem;
  --spacing-md: 1.5rem;
  --spacing-lg: 2rem;
  --spacing-xl: 3rem;
  --line-height: 1.6;
  --max-width: 720px;
  --sidebar-width: 200px;
}

body {
  font-family: var(--font-sans);
  font-size: 16px;
  line-height: var(--line-height);
  color: var(--color-text);
  background: var(--color-bg);
}

.container {
  display: flex;
  min-height: 100vh;
}

.sidebar {
  width: var(--sidebar-width);
  padding: var(--spacing-lg);
  border-right: 1px solid var(--color-border);
  position: sticky;
  top: 0;
  height: 100vh;
  overflow-y: auto;
}

.logo {
  font-size: 1.125rem;
  font-weight: 600;
  color: var(--color-text);
  margin-bottom: var(--spacing-xs);
}

.site-description {
  font-size: 0.8rem;
  color: var(--color-text-light);
  line-height: 1.5;
  margin-bottom: var(--spacing-md);
}

.site-description a {
  color: var(--color-text-muted);
  text-decoration: none;
}

.site-description a:hover {
  color: var(--color-accent);
}

.nav-link {
  display: block;
  color: var(--color-text-muted);
  text-decoration: none;
  padding: var(--spacing-xs) 0;
  font-size: 0.85rem;
  transition: color 0.15s ease;
}

.nav-link:hover,
.nav-link.active {
  color: var(--color-accent);
}

.category-filters {
  margin-top: var(--spacing-sm);
  padding-top: var(--spacing-sm);
  border-top: 1px solid var(--color-border);
}

.category-link {
  display: flex;
  justify-content: space-between;
  align-items: center;
  color: var(--color-text-muted);
  text-decoration: none;
  padding: 0.35rem 0;
  font-size: 0.8rem;
  cursor: pointer;
}

.category-link:hover,
.category-link.active {
  color: var(--color-accent);
}

.category-count {
  color: var(--color-text-light);
  font-size: 0.75rem;
}

.main {
  flex: 1;
  max-width: var(--max-width);
  padding: var(--spacing-lg);
  margin: 0 auto;
}

.add-paper {
  margin-bottom: var(--spacing-xl);
}

.add-paper-form {
  display: flex;
  gap: 0;
}

.url-input {
  flex: 1;
  padding: 0.625rem 0.875rem;
  font-size: 0.9rem;
  font-family: var(--font-sans);
  border: 1px solid var(--color-text);
  border-right: none;
  outline: none;
  background: var(--color-bg);
  color: var(--color-text);
}

.url-input::placeholder {
  color: var(--color-text-light);
}

.url-input:focus {
  border-color: var(--color-text);
}

.submit-btn {
  padding: 0.625rem 1.25rem;
  background: var(--color-text);
  color: var(--color-bg);
  border: 1px solid var(--color-text);
  font-size: 0.7rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  cursor: pointer;
  transition: background 0.15s ease;
}

.submit-btn:hover {
  background: #000;
}

.submit-btn:disabled {
  opacity: 0.6;
  cursor: not-allowed;
}

.form-status {
  margin-top: var(--spacing-xs);
  font-size: 0.8rem;
  min-height: 1.25rem;
}

.form-status.loading {
  color: var(--color-text-muted);
}

.form-status.error {
  color: #dc3545;
}

.form-status.success {
  color: #28a745;
}

.month-group {
  margin-bottom: var(--spacing-sm);
}

.month-header {
  font-size: 0.7rem;
  font-weight: 600;
  color: var(--color-text-light);
  text-transform: uppercase;
  letter-spacing: 0.08em;
  margin-bottom: 0.5rem;
  padding-bottom: 0.25rem;
  border-bottom: 1px solid var(--color-border);
}

.paper-card {
  padding: 0.5rem 0;
  border-bottom: 1px solid var(--color-border);
}

.paper-card:last-child {
  border-bottom: none;
}

.paper-header {
  cursor: pointer;
}

.paper-title {
  font-size: 0.875rem;
  font-weight: 600;
  color: var(--color-text);
  margin-bottom: 0.2rem;
  line-height: 1.35;
}

.paper-title:hover {
  color: var(--color-accent);
}

.paper-meta {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 0.5rem;
  font-size: 0.75rem;
  color: var(--color-text-muted);
}

.paper-authors {
  max-width: 50%;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.paper-date,
.paper-citations {
  color: var(--color-text-light);
  font-size: 0.7rem;
}

.paper-category-tag {
  display: inline-block;
  padding: 0.1rem 0.4rem;
  background: var(--color-bg-hover);
  color: var(--color-text-muted);
  font-size: 0.65rem;
  border-radius: 2px;
}

.paper-expanded {
  display: none;
  margin-top: var(--spacing-sm);
  padding-top: var(--spacing-sm);
  border-top: 1px solid var(--color-border);
}

.paper-card.expanded .paper-expanded {
  display: block;
}

.paper-summary {
  font-size: 0.875rem;
  color: var(--color-text);
  margin-bottom: var(--spacing-sm);
  line-height: 1.65;
}

.paper-abstract {
  color: var(--color-text-muted);
  font-size: 0.825rem;
  line-height: 1.6;
}

.paper-tags {
  display: flex;
  flex-wrap: wrap;
  gap: 0.375rem;
  margin-bottom: var(--spacing-sm);
}

.tag {
  display: inline-block;
  padding: 0.15rem 0.5rem;
  background: var(--color-bg-hover);
  color: var(--color-text-muted);
  font-size: 0.7rem;
  border-radius: 2px;
}

.paper-details {
  font-size: 0.8rem;
  color: var(--color-text-muted);
}

.paper-details p {
  margin-bottom: 0.35rem;
}

.paper-details a {
  color: var(--color-accent);
  text-decoration: none;
}

.paper-details a:hover {
  text-decoration: underline;
}

.paper-actions {
  margin-top: var(--spacing-sm);
  padding-top: var(--spacing-sm);
  border-top: 1px solid var(--color-border);
}

.delete-btn {
  background: none;
  border: none;
  color: var(--color-text-light);
  font-size: 0.75rem;
  cursor: pointer;
  padding: 0;
}

.delete-btn:hover {
  color: #dc3545;
}

.empty-state {
  text-align: center;
  padding: var(--spacing-xl);
  color: var(--color-text-muted);
}

.empty-state h2 {
  font-size: 1rem;
  font-weight: 500;
  margin-bottom: var(--spacing-xs);
}

.empty-state p {
  font-size: 0.875rem;
}

.loading-spinner {
  display: inline-block;
  width: 0.875rem;
  height: 0.875rem;
  border: 2px solid var(--color-border);
  border-top-color: var(--color-text);
  border-radius: 50%;
  animation: spin 0.8s linear infinite;
  vertical-align: middle;
  margin-left: 0.5rem;
}

@keyframes spin {
  to { transform: rotate(360deg); }
}

/* Password modal */
.modal-overlay {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background: rgba(0, 0, 0, 0.4);
  display: flex;
  align-items: center;
  justify-content: center;
  z-index: 1000;
}

.modal {
  background: var(--color-bg);
  padding: var(--spacing-md);
  max-width: 320px;
  width: 90%;
}

.modal h3 {
  font-size: 0.9rem;
  font-weight: 600;
  margin-bottom: var(--spacing-sm);
}

.modal input {
  width: 100%;
  padding: 0.625rem 0.875rem;
  font-size: 0.9rem;
  font-family: var(--font-sans);
  border: 1px solid var(--color-text);
  margin-bottom: var(--spacing-sm);
  outline: none;
}

.modal-buttons {
  display: flex;
  gap: var(--spacing-xs);
  justify-content: flex-end;
}

.modal-btn {
  padding: 0.5rem 1rem;
  font-size: 0.7rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  cursor: pointer;
  border: 1px solid var(--color-text);
}

.modal-btn-primary {
  background: var(--color-text);
  color: var(--color-bg);
}

.modal-btn-secondary {
  background: var(--color-bg);
  color: var(--color-text);
}

.modal-error {
  color: #dc3545;
  font-size: 0.75rem;
  margin-bottom: var(--spacing-xs);
}

@media (max-width: 768px) {
  .container {
    flex-direction: column;
  }

  .sidebar {
    width: 100%;
    height: auto;
    position: static;
    border-right: none;
    border-bottom: 1px solid var(--color-border);
    padding: var(--spacing-sm);
  }

  .logo {
    margin-bottom: 0.25rem;
  }

  .site-description {
    margin-bottom: var(--spacing-sm);
  }

  .category-filters {
    display: flex;
    flex-wrap: wrap;
    gap: var(--spacing-xs);
    margin-top: var(--spacing-xs);
    padding-top: var(--spacing-xs);
  }

  .category-link {
    padding: 0.2rem 0.4rem;
    background: var(--color-bg-hover);
    border-radius: 2px;
  }

  .main {
    padding: var(--spacing-sm);
  }

  .add-paper-form {
    flex-direction: column;
    gap: var(--spacing-xs);
  }

  .url-input {
    border-right: 1px solid var(--color-text);
  }

  .submit-btn {
    width: 100%;
    padding: 0.75rem;
  }

  .paper-authors {
    max-width: 100%;
  }
}