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

resources/views/admin/pages/_form.blade.php

@@ -98,6 +98,19 @@
                 @enderror
             </div>
         </div>
+        <div class="mt-4">
+            <label class="inline-flex cursor-pointer items-center gap-2 text-sm text-slate-800">
+                <input
+                    type="checkbox"
+                    name="background_fixed"
+                    value="1"
+                    class="rounded border-slate-300 text-indigo-600 focus:ring-indigo-500"
+                    @checked(old('background_fixed', $page?->background_fixed ?? false))
+                />
+                {{ __('Fix background to viewport') }}
+            </label>
+            <p class="mt-1 text-xs text-slate-600">{{ __('When enabled, the background image and overlay stay fixed while visitors scroll long content.') }}</p>
+        </div>
     </div>
 
     <div class="grid gap-6 sm:grid-cols-2">