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/purchase_confirmation.html.erb

@@ -5,7 +5,7 @@
   </div>
 
   <div style="background-color: white; border-radius: 8px; padding: 30px; margin: 20px 0; box-shadow: 0 2px 4px rgba(0,0,0,0.1);">
-    <% if user.first_name %>
+    <% if @user.first_name %>
       <h2 style="color: #212529; margin-top: 0;">Bonjour <%= @user.first_name %>,</h2>
     <% else %>
       <h2 style="color: #212529; margin-top: 0;">Bonjour <%= @user.email.split('@').first %>,</h2>