fix: show descriptive message when non-admin accesses admin UI

Author: jaspermayone

app/controllers/application_controller.rb

@@ -12,6 +12,14 @@ def after_sign_in_path_for(resource)
     current_user.admin_access? ? admin_root_path : dashboard_root_path
   end
 
+  def admin_unauthorized
+    if user_signed_in?
+      redirect_to dashboard_root_path, alert: "You don't have permission to access that page."
+    else
+      redirect_to new_user_session_path
+    end
+  end
+
   private
 
   def user_not_authorized

config/routes.rb

@@ -201,8 +201,8 @@
   end
 
   # Fallback if AdminConstraint fails
-  get "admin",       to: redirect("/users/sign_in")
-  get "admin/*path", to: redirect("/users/sign_in")
+  get "admin",       to: "application#admin_unauthorized"
+  get "admin/*path", to: "application#admin_unauthorized"
 
   get "unauthorized", to: "errors#unauthorized"
   get "404",          to: "errors#not_found"