FIX: Harmonise category body class generation on server/client (#16967)

The server-side implementation had unintentionally changed to include `-{id}` at the end of the body class name. This change meant that the JS client was unaware of the class, and didn't remove it when navigating away from the category page.

This commit fixes the server-side implementation to match the client

app/helpers/application_helper.rb

@@ -181,7 +181,7 @@ module ApplicationHelper
     result = ApplicationHelper.extra_body_classes.to_a
 
     if @category && @category.url.present?
-      result << "category-#{@category.url.sub(/^\/c\//, '').gsub(/\//, '-')}"
+      result << "category-#{@category.slug_path.join('-')}"
     end
 
     if current_user.present? &&