Improved spacing of the publish page on small screens

fixes #24898

This PR improves the right spacing of the publish page header, by making
it proportional to the left one. It also ensures that the "Back to
settings" secondary CTA does not overflow horizontally on very narrow
viewports.

Author: lorumic

ghost/admin/app/styles/components/publishmenu.css

@@ -231,8 +231,7 @@
         align-items: center;
         height: 64px;
         margin: 0;
-        padding: 0;
-        padding-left: 15px;
+        padding: 0 15px;
         background-color: var(--white);
         border-radius: 0;
     }
@@ -649,8 +648,8 @@
 
 .gh-publish-cta {
     display: flex;
-    width: max-content;
     gap: 3.2rem;
+    width: 100%;
 }
 
 .gh-publish-cta .gh-btn {