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>

scss/westgate/_topbar.scss

@@ -296,6 +296,12 @@ $wg-topbar-panel-bg: rgba(21, 17, 26, 0.98);
   outline: 0;
 }
 
+// The input's own outline is off; put the visible focus ring on its wrapper.
+.wg-topbar__drawer-search:focus-within {
+  border-color: rgba(194, 163, 90, 0.5);
+  box-shadow: 0 0 0 1px var(--wg-focus);
+}
+
 .wg-topbar__drawer-nav,
 .wg-topbar__drawer-actions,
 .wg-topbar__drawer-auth {