feat(ux): first-time onboarding overlay for new users

Shows a 3-screen modal overlay on first dashboard visit explaining key
features; dismissed state is persisted to localStorage so it never
reappears.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/styles/dashboard.css

@@ -1220,3 +1220,98 @@
   opacity: 0.3;
   cursor: not-allowed;
 }
+
+/* ── Onboarding Overlay ── */
+.onboarding-overlay {
+  position: fixed;
+  inset: 0;
+  z-index: var(--z-modal);
+  background: color-mix(in srgb, var(--color-bg) 70%, transparent);
+  backdrop-filter: var(--blur-lg);
+  -webkit-backdrop-filter: var(--blur-lg);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: var(--space-6);
+  animation: onboarding-in 0.3s var(--ease-out);
+}
+
+.onboarding-overlay--out {
+  animation: onboarding-out 0.25s ease forwards;
+}
+
+@keyframes onboarding-in {
+  from { opacity: 0; }
+  to   { opacity: 1; }
+}
+
+@keyframes onboarding-out {
+  from { opacity: 1; }
+  to   { opacity: 0; }
+}
+
+.onboarding-card {
+  background: var(--color-surface);
+  border: 1px solid var(--color-border-subtle);
+  border-radius: var(--radius-xl);
+  padding: var(--space-8);
+  max-width: 360px;
+  width: 100%;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: var(--space-4);
+  text-align: center;
+  box-shadow: var(--shadow-lg);
+}
+
+.onboarding-icon {
+  width: 48px;
+  height: 48px;
+  color: var(--color-accent);
+}
+
+.onboarding-title {
+  font-size: var(--text-xl);
+  font-weight: var(--font-weight-semibold);
+  color: var(--color-text);
+  margin: 0;
+}
+
+.onboarding-body {
+  font-size: var(--text-base);
+  color: var(--color-text-secondary);
+  line-height: 1.6;
+  margin: 0;
+}
+
+.onboarding-dots {
+  display: flex;
+  gap: var(--space-2);
+}
+
+.onboarding-dot {
+  width: 8px;
+  height: 8px;
+  border-radius: 50%;
+  background: var(--color-border);
+  transition: background 0.2s;
+}
+
+.onboarding-dot--active {
+  background: var(--color-accent);
+}
+
+.onboarding-actions {
+  display: flex;
+  gap: var(--space-3);
+  width: 100%;
+  justify-content: flex-end;
+  margin-top: var(--space-2);
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .onboarding-overlay,
+  .onboarding-overlay--out { animation: none; }
+  .onboarding-dot { transition: none; }
+}