Fix Devise Turbo Stream responses for flash messages

Wire Users::DeviseController as Devise parent controller with the
Turbo-aware responder, so failed form submissions (login, password
reset) render correctly as HTML with 422 status instead of
unprocessable turbo-stream content that Turbo silently drops.

Also fixes a NameError in the responder (rendering_options ->
error_rendering_options) and removes the dead TurboController.

config/initializers/devise.rb

@@ -6,6 +6,7 @@ require 'securerandom'
 # Many of these configuration options can be set straight in your model.
 Devise.setup do |config|
   # Hotwire/Turbo
+  config.parent_controller = 'Users::DeviseController'
   config.responder.error_status = :unprocessable_entity
   config.responder.redirect_status = :see_other
 
@@ -324,3 +325,10 @@ Devise.setup do |config|
   # changed. Defaults to true, so a user is signed in automatically after changing a password.
   # config.sign_in_after_change_password = true
 end
+
+# Turbo-aware responder for Devise controllers
+ActiveSupport.on_load(:devise_controller) do
+  self.responder = Users::DeviseController::Responder
+  Users::DeviseController::Responder.error_status = :unprocessable_entity
+  Users::DeviseController::Responder.redirect_status = :see_other
+end