Theme and nav improvements: lang switcher styling, dropdown, theme thumbnail

- Style language switcher dropdown to match site colours
- Move language switcher left of social icons
- Set theme thumbnail to logo PNG in blueprints
- Add Dutch language override and system config tweaks
- Various CSS fixes for dropdown alignment and width

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

themes/natascha/templates/partials/langswitcher.html.twig

@@ -1,7 +1,7 @@
 {% extends 'partials/langswitcher-logic.html.twig' %}
 
 {% block language_block_start %}
-<li class="nav-item dropdown">
+<li class="nav-item dropdown d-flex align-items-center">
     <a class="nav-link dropdown-toggle p-1" href="#" role="button" data-bs-toggle="dropdown" aria-expanded="false">
         {{ langswitcher.current|upper }}
     </a>