Fix wrong width on content warnings and filters in web UI (#31761)

app/javascript/styles/mastodon/components.scss

@@ -10916,6 +10916,7 @@ noscript {
 }
 
 .content-warning {
+  box-sizing: border-box;
   background: rgba($ui-highlight-color, 0.05);
   color: $secondary-text-color;
   border-top: 1px solid;