feat: optional fixed viewport background on public pages

Adds background_fixed column, admin checkbox, fixed-position layers on the public layout, Dutch strings, and store tests.

Made-with: Cursor

app/Models/PreregistrationPage.php

@@ -32,6 +32,7 @@ class PreregistrationPage extends Model
         'background_image',
         'background_overlay_color',
         'background_overlay_opacity',
+        'background_fixed',
         'logo_image',
         'is_active',
     ];
@@ -42,6 +43,7 @@ class PreregistrationPage extends Model
             'start_date' => 'datetime',
             'end_date' => 'datetime',
             'phone_enabled' => 'boolean',
+            'background_fixed' => 'boolean',
             'is_active' => 'boolean',
         ];
     }