chore: update default email in application mailer

- Change default from email to a placeholder value
- This avoids using sensitive environment variables during development

This change ensures that the application does not expose sensitive
information while still allowing for email functionality in a
development environment.

app/mailers/application_mailer.rb

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