feat(a11y): WCAG AA polish on the site onboarding flow

- index.css: add visible :focus-visible rings for .site-card-button
  and .site-button so keyboard users can see where focus lives
- SiteCard: drop outline:none, add aria-pressed and aria-label so
  screen readers announce selection state
- SiteButton: add the .site-button class for the shared focus ring
- SiteDisclaimerScreen: keyboard skip via Enter / Space / Escape, a
  role="region" + aria-label wrapper and aria-live="polite" on the
  message; honour prefers-reduced-motion on the fade
- IntroVideoPlayer: role="region" with a skip hint in aria-label,
  preload="auto", and aria-hidden on the decorative caption span

src/index.css

@@ -44,6 +44,18 @@ select {
   font: inherit;
 }
 
+/* Site onboarding — accessible focus rings (WCAG 2.4.7) */
+.site-card-button:focus-visible,
+.site-button:focus-visible {
+  outline: 3px solid #ffffff;
+  outline-offset: 3px;
+  box-shadow: 0 0 0 5px rgba(0, 0, 0, 0.55);
+}
+
+.site-card-button[aria-pressed="true"]:focus-visible {
+  outline-color: #a8d5a2;
+}
+
 canvas {
   display: block;
 }