fix: update default from email in ApplicationMailer

- Change default from email to use ENV variable
- Fetch SMTP username from Rails credentials

This change improves the security of the application by avoiding
hardcoded email addresses and allows for easier configuration
across different environments.

app/mailers/application_mailer.rb

@@ -1,4 +1,4 @@
 class ApplicationMailer < ActionMailer::Base
-  default from: "noreply@mail.frytea.com"
+  default from: ENV.fetch("RAILS_SMTP_USERNAME", Rails.application.credentials.dig(:smtp, :user_name))
   layout "mailer"
 end