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/Http/Requests/Admin/ValidatesPreregistrationPageInput.php

@@ -23,6 +23,7 @@ trait ValidatesPreregistrationPageInput
             'post_submit_redirect_url' => ['nullable', 'string', 'url:http,https', 'max:500'],
             'background_overlay_color' => ['nullable', 'string', 'regex:/^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$/'],
             'background_overlay_opacity' => ['nullable', 'integer', 'min:0', 'max:100'],
+            'background_fixed' => ['sometimes', 'boolean'],
             'page_background' => ['nullable', 'file', 'image', 'mimes:jpeg,png,jpg,webp', 'max:5120'],
             'remove_page_background' => ['sometimes', 'boolean'],
             'start_date' => ['required', 'date'],
@@ -69,6 +70,7 @@ trait ValidatesPreregistrationPageInput
 
         $this->merge([
             'is_active' => $this->boolean('is_active'),
+            'background_fixed' => $this->boolean('background_fixed'),
             'remove_page_background' => $this->boolean('remove_page_background'),
             'ticketshop_url' => $ticketshopNormalized,
             'post_submit_redirect_url' => $redirectNormalized,