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/Controllers/Api/V1/PublicRegistrationDataController.php

@@ -60,6 +60,9 @@ final class PublicRegistrationDataController extends Controller
                     'start_date' => $festivalEvent->start_date->toDateString(),
                     'end_date' => $festivalEvent->end_date->toDateString(),
                     'organisation_id' => $festivalEvent->organisation_id,
+                    'registration_banner_url' => $festivalEvent->registration_banner_url,
+                    'registration_welcome_text' => $festivalEvent->registration_welcome_text,
+                    'registration_logo_url' => $festivalEvent->registration_logo_url,
                 ],
                 'sections' => $sections->map(fn (FestivalSection $section) => [
                     'id' => $section->id,