Fix flash messages coloring

Previously, all flash messages were showing with the same (black)
background. Internally, the flash object converts and stores keys
as strings [1].

[1] https://github.com/rails/rails/blob/main/actionpack/lib/action_dispatch/middleware/flash.rb#L163

admin/app/views/layouts/solidus_admin/application.html.erb

@@ -29,7 +29,7 @@
 
     <div class="fixed inset-x-0 bottom-3 flex items-center justify-center flex-col gap-3 pointer-events-none" role="alert">
       <% flash.each do |key, message| %>
-        <%= render component("ui/toast").new(text: message, scheme: key == :error ? :error : :default) %>
+        <%= render component("ui/toast").new(text: message, scheme: key.to_sym == :error ? :error : :default) %>
       <% end %>
     </div>
   </body>