feat: more work on UI app

Co-authored-by: Copilot <copilot@github.com>

lib/bds/desktop/shell_controller.ex

@@ -2,6 +2,6 @@ defmodule BDS.Desktop.ShellController do
   @moduledoc false
 
   def index_html do
-    File.read!(Application.app_dir(:bds, ["priv", "ui", "index.html"]))
+    BDS.UI.ShellPage.render()
   end
 end