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>

plugin.json

@@ -9,6 +9,7 @@
     { "hook": "filter:user.saveSettings", "method": "saveUserSettings" },
     { "hook": "filter:user.profileMenu", "method": "addProfileItem" },
     { "hook": "filter:middleware.renderHeader", "method": "filterMiddlewareRenderHeader" },
+    { "hook": "filter:middleware.renderFooter", "method": "filterMiddlewareRenderFooter" },
     { "hook": "filter:user.iconBackgrounds", "method": "getIconBackgrounds" }
   ],
   "scripts": ["public/client.js", "../nodebb-theme-harmony/public/harmony.js"],