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>

config/site.yaml

@@ -0,0 +1,19 @@
+title: 'Kunstgalerie Natascha Rieter'
+default_lang: nl
+author:
+  name: 'Natascha Rieter'
+  email: info@natascha-rieter.nl
+taxonomies:
+  - category
+  - tag
+metadata:
+  description: 'My Grav Site'
+summary:
+  enabled: true
+  format: short
+  size: 300
+  delimiter: '==='
+redirects: null
+routes: null
+blog:
+  route: /blog