Show validation errors on sign up form

Display error messages when registration fails (e.g. duplicate username)
using Devise 5's shared error_messages partial.

Author: davidsf

app/views/devise/registrations/new.html.haml

@@ -6,6 +6,7 @@
           %h3.panel-title
             = t('Sign Up')
         .panel-body
+          = render "devise/shared/error_messages", resource: resource
           = form_for(resource, as: resource_name, url: registration_path(resource_name)) do |f|
             = render partial: 'form_fields', locals: { f: f }