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

app/views/ticket_mailer/event_reminder.html.erb

@@ -1,6 +1,6 @@
 <div style="font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif; max-width: 600px; margin: 0 auto; padding: 20px; background-color: #f8f9fa; border-radius: 8px;">
   <div style="text-align: center; padding: 20px 0; border-bottom: 1px solid #e9ecef;">
-    <h1 style="color: #4c1d95; margin: 0; font-size: 28px;">ApéroNight</h1>
+    <h1 style="color: #4c1d95; margin: 0; font-size: 28px;"><%= ENV.fetch("APP_NAME", "Aperonight") %></h1>
     <p style="color: #6c757d; margin: 10px 0 0;">Rappel d'événement</p>
   </div>