Theme post code blocks + a11y fixes (impeccable audit) (#25)

Code blocks in posts rendered with the markdown plugin's default light highlight.js theme — white blocks on the dark ledger. This adds dark velvet pre/code styling and a Westgate-toned syntax palette (gold keywords, ledger-ink titles, muted green strings) for post content and the composer preview.

Also picks up the quick wins from an impeccable audit:

- Visible focus ring on the mobile drawer search (input had `outline: 0` with no replacement)
- Input placeholder contrast raised to pass WCAG 4.5:1; tagsinput placeholder reuses the token instead of `$gray-500`
- `prefers-reduced-motion` block zeroing transitions/animations (there were none before)
- Fixed a stray `</li>` closing a `<div>` in `quick-search-results.tpl`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #25
Co-authored-by: vickydotbat <vickydotbat@tutamail.com>
Co-committed-by: vickydotbat <vickydotbat@tutamail.com>

templates/partials/category/selector-dropdown-content.tpl

@@ -24,7 +24,7 @@
         {{{ each categoryItems }}}
         <li role="presentation" class="category {{{ if ./disabledClass }}}disabled {{{ end }}}{{{ if ./class }}} {./class}{{{ end }}}" data-cid="{./cid}" data-name="{./name}" data-parent-cid="{./parentCid}">
             <a class="dropdown-item rounded-1 {{{ if ./disabledClass }}}disabled{{{ end }}}" role="menuitem" href="#">{./level}
-                <span component="category-markup" style="{{{ if ./match }}}font-weight: bold;{{{end}}}">
+                <span component="category-markup" class="{{{ if ./match }}}fw-bold{{{end}}}">
                     <span class="category-item d-inline-flex align-items-center gap-1 {{{ if ./class }}}{./class}{{{ end }}}">
                         {{{ if ./icon }}}
                         {{buildCategoryIcon(@value, "24px", "rounded-circle")}}