feat: event registration branding with vertical wizard layout

- Add registration_banner_url, registration_welcome_text, registration_logo_url
  columns to events table with migration
- Add uploadImage endpoint (POST .../upload-image) with form request validation
  for banner and logo images (jpg/png/webp, max 5MB)
- Include branding fields in EventResource and PublicRegistrationDataController
- Build registration settings UI in organizer event settings page with
  banner/logo upload and welcome text editor
- Redesign portal registration page: hero banner with gradient overlay,
  welcome text card, vertical step navigation (desktop) / horizontal chips
  (mobile), two-column form fields with density="comfortable"
- Update success page with event banner and consistent branding
- Seed welcome text for Echt Feesten 2026
- Add 9 PHPUnit tests covering image upload, branding fields in API responses

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

api/database/seeders/DevSeeder.php

@@ -175,6 +175,7 @@ class DevSeeder extends Seeder
                 'event_type' => 'festival',
                 'event_type_label' => 'Festival',
                 'sub_event_label' => 'Programmaonderdeel',
+                'registration_welcome_text' => 'Wij zoeken enthousiaste vrijwilligers voor Echt Feesten 2026! Word onderdeel van ons team en beleef het festival van de andere kant. Gratis toegang, maaltijden, en een onvergetelijke ervaring.',
             ]);
 
             $vrijdag = Event::create([