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

_includes/components/widgets/toc.njk

@@ -3,7 +3,7 @@
 <is-land on:visible>
 <div class="widget">
   <h3 class="widget-title">Contents</h3>
-  <nav class="toc">
+  <nav class="toc" aria-label="Table of contents">
     <ul class="space-y-1 text-sm">
       {% for item in toc %}
       <li class="{% if item.level > 2 %}ml-{{ (item.level - 2) * 3 }}{% endif %}">