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/topics_list.tpl

@@ -15,7 +15,7 @@
 				</a>
 				{{{ if showSelect }}}
 				<div class="checkbox position-absolute top-100 start-50 translate-middle-x pt-2 m-0 d-none d-lg-flex" style="max-width:max-content">
-					<i component="topic/select" class="fa text-muted pointer fa-square-o p-1 hover-visible"></i>
+					<i component="topic/select" class="fa text-muted pointer fa-square-o p-1 hover-visible" role="checkbox" aria-checked="false" tabindex="0" aria-label="[[westgate:select-topic]]"></i>
 				</div>
 				{{{ end }}}
 			</div>
@@ -73,7 +73,7 @@
 				</span>
 				{{{ if showSelect }}}
 				<div class="checkbox position-absolute top-0 end-0 m-0 d-flex d-lg-none" style="max-width:max-content">
-					<i component="topic/select" class="fa fa-square-o text-muted pointer p-1"></i>
+					<i component="topic/select" class="fa fa-square-o text-muted pointer p-1" role="checkbox" aria-checked="false" tabindex="0" aria-label="[[westgate:select-topic]]"></i>
 				</div>
 				{{{ end }}}
 			</div>