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

lib/unfurl-shortcode.js

@@ -109,8 +109,8 @@ export function renderCard(url, metadata) {
   <a href="${escapeHtml(url)}" rel="noopener" target="_blank" class="flex no-underline text-inherit hover:text-inherit">
     <div class="flex-1 p-3 sm:p-4 min-w-0">
       <p class="font-semibold text-sm sm:text-base text-surface-900 dark:text-surface-100 truncate m-0">${escapeHtml(title)}</p>
-      ${desc ? `<p class="text-xs sm:text-sm text-surface-500 dark:text-surface-400 mt-1 m-0 line-clamp-2">${escapeHtml(desc)}</p>` : ""}
-      <p class="text-xs text-surface-400 dark:text-surface-500 mt-2 m-0">${faviconHtml}${escapeHtml(domain)}</p>
+      ${desc ? `<p class="text-xs sm:text-sm text-surface-600 dark:text-surface-400 mt-1 m-0 line-clamp-2">${escapeHtml(desc)}</p>` : ""}
+      <p class="text-xs text-surface-400 dark:text-surface-400 mt-2 m-0">${faviconHtml}${escapeHtml(domain)}</p>
     </div>
     ${imgHtml}
   </a>