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/app/Http/Resources/Api/V1/EventResource.php

@@ -27,6 +27,9 @@ final class EventResource extends JsonResource
             'event_type_label' => $this->event_type_label,
             'sub_event_label' => $this->sub_event_label,
             'is_recurring' => $this->is_recurring,
+            'registration_banner_url' => $this->registration_banner_url,
+            'registration_welcome_text' => $this->registration_welcome_text,
+            'registration_logo_url' => $this->registration_logo_url,
             'is_festival' => $this->resource->isFestival(),
             'is_sub_event' => $this->resource->isSubEvent(),
             'is_flat_event' => $this->resource->isFlatEvent(),