Fix invitation list (use dedicated variable), invitation email subject and content

- Pass invitations from controller instead of Twig filter on paginator
- Email subject: "Votre invitation" for invitations, "Vos billets" for purchases
- Email content: different intro text for invitations

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Controller/AccountController.php

@@ -408,6 +408,7 @@ class AccountController extends AbstractController
             'commission_rate' => $user->getCommissionRate() ?? 0,
             'billet_design' => $em->getRepository(BilletDesign::class)->findOneBy(['event' => $event]),
             'event_orders' => $eventOrders,
+            'invitations' => $em->getRepository(BilletBuyer::class)->findBy(['event' => $event, 'totalHT' => 0], ['createdAt' => 'DESC']),
             'event_total_ht' => $eventTotalHT / 100,
             'event_total_sold' => $eventTotalSold,
             'event_total_orders' => \count($paidEventOrders),