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>

user/themes/natascha/templates/partials/nav.html.twig

@@ -17,6 +17,7 @@
                 </li>
                 {% endif %}
                 {% endfor %}
+                {% include 'partials/langswitcher.html.twig' %}
                 <li class="nav-item">
                     <a class="nav-link p-1" href="#" aria-label="Instagram">
                         <img src="{{ url('theme://images/icon-instagram.png') }}" height="25" class="social-icon" alt="Instagram">
@@ -32,7 +33,6 @@
                         <img src="{{ url('theme://images/icon-linked-in.png') }}" height="25" class="social-icon" alt="LinkedIn">
                     </a>
                 </li>
-                {% include 'partials/langswitcher.html.twig' %}
             </ul>
         </div>
     </div>