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/controllers/onboarding_controller.rb

@@ -11,7 +11,7 @@ class OnboardingController < ApplicationController
       current_user.update!(onboarding_params)
       current_user.complete_onboarding!
       
-      flash[:notice] = "Bienvenue sur AperoNight ! Votre profil a été configuré avec succès."
+      flash[:notice] = "Bienvenue sur #{Rails.application.config.app_name} ! Votre profil a été configuré avec succès."
       redirect_to dashboard_path
     else
       flash.now[:alert] = "Veuillez remplir tous les champs requis."

app/views/onboarding/index.html.erb

@@ -71,7 +71,7 @@
             <p class="text-sm text-gray-500">
               Vous pourrez modifier ces informations plus tard.
             </p>
-            <%= form.submit "Finaliser mon profil",
+            <%= form.submit "Compléter mon profil",
                 class: "w-full px-8 py-3 bg-purple-600 text-white font-semibold rounded-lg hover:bg-purple-700 focus:ring-2 focus:ring-purple-500 focus:ring-offset-2 transition-colors cursor-pointer" %>
           </div>
         </div>

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>
 

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>