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/routes/api.php

@@ -77,6 +77,7 @@ Route::middleware('auth:sanctum')->group(function () {
         ->only(['index', 'show', 'store', 'update', 'destroy']);
     Route::get('organisations/{organisation}/events/{event}/children', [EventController::class, 'children']);
     Route::post('organisations/{organisation}/events/{event}/transition', [EventController::class, 'transition']);
+    Route::post('organisations/{organisation}/events/{event}/upload-image', [EventController::class, 'uploadImage']);
 
     // Organisation-scoped resources
     Route::prefix('organisations/{organisation}')->group(function () {