SiteHeader: Set the bullet in ::before instead of ::after

Set the bullet between nav items in the ::before content rather than the ::after.
I found this lets me control spacing better.

Set the flex gap to XS spacing.

assets/css/099_site_header.css

@@ -28,6 +28,7 @@
 
     ul {
       display: flex;
+      gap: var(--space-xs);
       list-style: none;
       padding-inline: 0;
 
@@ -36,7 +37,7 @@
         padding: 0;
       }
 
-      li:not(:last-child)::after {
+      li:not(:first-child)::before {
         content: "•";
       }
     }