Theme code blocks in posts; a11y fixes from impeccable audit

- Dark velvet pre/code styling + Westgate hljs syntax palette for post
  content and composer preview (markdown plugin's light hljs theme was
  rendering white blocks)
- Visible focus ring on mobile drawer search (was outline: 0 with no
  replacement)
- Placeholder contrast raised to pass 4.5:1; tagsinput placeholder now
  reuses the token instead of $gray-500
- prefers-reduced-motion block zeroing transitions/animations
- Fix stray </li> closing a <div> in quick-search-results.tpl

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

templates/partials/quick-search-results.tpl

@@ -45,6 +45,6 @@
 {{{ end }}}
 
 {{{if !posts.length}}}
-<div class="text-center no-results py-2 py-md-0">[[search:no-matches]]</li>
+<div class="text-center no-results py-2 py-md-0">[[search:no-matches]]</div>
 <li role="separator" class="dropdown-divider d-block d-md-none"></li>
 {{{end}}}