Fix click access to scrollbar when design zooming

pb-4 -> mb-4 so that the horizontal scrollbar is available to be clicked

ee/app/assets/javascripts/design_management/pages/design/index.vue

@@ -237,7 +237,7 @@ export default {
           :scale="scale"
           @openCommentForm="openCommentForm"
         />
-        <div class="design-scaler-wrapper position-absolute w-100 pb-4 d-flex-center">
+        <div class="design-scaler-wrapper position-absolute w-100 mb-4 d-flex-center">
           <design-scaler @scale="scale = $event" />
         </div>
       </div>

ee/changelogs/unreleased/design-zooming-scrollbar-fix.yml

+---
+title: Fix access to horizontal scrollbar when zooming in on a design
+merge_request:
+author:
+type: fixed

ee/spec/frontend/design_management/pages/design/__snapshots__/index_spec.js.snap

@@ -23,7 +23,7 @@ exports[`Design management design index page renders design index 1`] = `
     />
      
     <div
-      class="design-scaler-wrapper position-absolute w-100 pb-4 d-flex-center"
+      class="design-scaler-wrapper position-absolute w-100 mb-4 d-flex-center"
     >
       <design-scaler-stub />
     </div>
@@ -117,7 +117,7 @@ exports[`Design management design index page with error GlAlert is rendered in c
     />
      
     <div
-      class="design-scaler-wrapper position-absolute w-100 pb-4 d-flex-center"
+      class="design-scaler-wrapper position-absolute w-100 mb-4 d-flex-center"
     >
       <design-scaler-stub />
     </div>