[#72945] Add BorderBoxList transparent scheme

Adds `scheme: :transparent` to BorderBoxListComponent with CSS overrides
for transparent header background and no separator line. Backlogs
callers adopt the transparent scheme.

https://community.openproject.org/wp/72945

lookbook/previews/open_project/common/border_box_list_component_preview.rb

@@ -61,6 +61,35 @@ module OpenProject
         end
       end
 
+      # @label Transparent scheme
+      # @param interactive toggle
+      # @param collapsible [Boolean] toggle
+      def transparent(padding: :default, interactive: false, collapsible: false)
+        render OpenProject::Common::BorderBoxListComponent.new(
+          container: "border-box-list-transparent-preview",
+          scheme: :transparent,
+          interactive: boolean_preview_param(interactive),
+          collapsible:
+        ) do |list|
+          list.with_header(title: "Sprint backlog", count: true) do |header|
+            header.with_description { "3 points remaining" }
+            header.with_action_button do |button|
+              button.with_leading_visual_icon(icon: :rocket)
+              "Start sprint"
+            end
+            header.with_menu(button_aria_label: "Sprint actions") do |menu|
+              menu.with_item(label: "Edit sprint") do |menu_item|
+                menu_item.with_leading_visual_icon(icon: :pencil)
+              end
+            end
+          end
+
+          list.with_item { "User authentication stories" }
+          list.with_item { "Dashboard improvements" }
+          list.with_item { "API documentation" }
+        end
+      end
+
       # @label With work package items
       # @param interactive toggle
       # @param collapsible toggle