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/blueprints.yaml

@@ -2,5 +2,6 @@ name: Natascha
 version: 1.0.0
 description: Custom theme for Natascha Rieter art gallery website
 type: theme
+thumbnail: /user/themes/natascha/images/logo-blauw.png
 author:
   name: Claude