Increase the width of the unread notification border. (#21692)

The smaller border is difficult to see for some users, especially
when the browser window was thinner, and so the unread border is at the
very left edge of the window.

app/javascript/styles/mastodon/components.scss

@@ -7666,7 +7666,7 @@ noscript {
       left: 0;
       width: 100%;
       height: 100%;
-      border-left: 2px solid $highlight-text-color;
+      border-left: 4px solid $highlight-text-color;
       pointer-events: none;
     }
   }