a11y: comprehensive WCAG 2.1 Level AA accessibility audit

- Add skip-to-main-content link and main content ID target
- Add prefers-reduced-motion media queries for all animations
- Enhance visible focus indicators (2px offset, high-contrast ring)
- Replace ~160 text-surface-500 instances with text-surface-600/dark:text-surface-400
  for 4.5:1+ contrast ratio compliance
- Add aria-hidden="true" to ~30+ decorative SVG icons across sidebars/widgets
- Convert facepile containers from div to semantic ul/li with role="list"
- Add aria-label to icon-only buttons (share, sort controls)
- Add sr-only labels to form inputs (webmention, search)
- Add aria-live="polite" to dynamically loaded webmentions
- Add aria-label with relative+absolute date to time-difference component
- Add keyboard handlers (Enter/Space) to custom interactive elements
- Add aria-label to nav landmarks (table of contents)
- Fix modal focus trap and dialog accessibility
- Fix lightbox keyboard navigation and screen reader announcements

Confab-Link: http://localhost:8080/sessions/edb1b7b0-da66-4486-bd9c-d1cfa7553b88

css/lite-yt-embed.css

@@ -70,8 +70,11 @@ lite-youtube > .lty-playbtn {
 }
 
 lite-youtube:hover > .lty-playbtn,
-lite-youtube .lty-playbtn:focus {
+lite-youtube .lty-playbtn:focus-visible {
     filter: none;
+    outline: 2px solid #fbbf24;
+    outline-offset: -2px;
+    border-radius: 4px;
 }
 
 /* Post-click styles */