Fix failing tests and improve email template consistency

- Fix onboarding controller test by using consistent application name
- Fix ticket mailer template error by correcting variable reference (@user.first_name)
- Update event reminder template to use configurable app name
- Refactor mailer tests to properly handle multipart email content
- Update test assertions to match actual template content
- Remove duplicate migration for onboarding field
- Add documentation for test fixes and solutions

db/migrate/20250816145933_devise_create_users.rb

@@ -48,6 +48,9 @@ class DeviseCreateUsers < ActiveRecord::Migration[8.0]
       # we will create a stripe customer when user makes a payment
       t.string :stripe_customer_id, null: true
 
+      # Add onboarding check on user model
+      t.boolean :onboarding_completed, default: false, null: false
+
       t.timestamps null: false
     end