Fix tenant event form package selector so it no longer renders empty-value options, handles loading/empty

states, and pulls data from the authenticated /api/v1/tenant/packages endpoint.
    (resources/js/admin/pages/EventFormPage.tsx, resources/js/admin/api.ts)
  - Harden tenant-admin auth flow: prevent PKCE state loss, scope out StrictMode double-processing, add SPA
    routes for /event-admin/login and /event-admin/logout, and tighten token/session clearing semantics (resources/js/admin/auth/{context,tokens}.tsx, resources/js/admin/pages/{AuthCallbackPage,LogoutPage}.tsx,
    resources/js/admin/router.tsx, routes/web.php)

app/Http/Controllers/CheckoutController.php

@@ -123,7 +123,9 @@ class CheckoutController extends Controller
             $user->sendEmailVerificationNotification();
 
             // Willkommens-E-Mail senden
-            Mail::to($user)->queue(new Welcome($user));
+            Mail::to($user)
+                ->locale($user->preferred_locale ?? app()->getLocale())
+                ->queue(new Welcome($user));
         });
 
         return response()->json([