feat: registration field polish, multi-category tags, file uploads, Partner icon

- Restructure field editor dialog: move Options section to bottom with
  divider and subheader, fix delete button with flex layout
- Change tag_category (single string) to tag_categories (JSON array)
  supporting multiple category selection in tag picker fields
- Portal tag picker now groups tags by category with subheaders
- Add generic file upload endpoint (FileUploadService + UploadController)
- Replace email branding logo URL text field with ImageUploadField
- Update Partner crowd type default icon to tabler-affiliate
- Apply changes consistently to both field and template dialogs

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

api/tests/Feature/Api/V1/PublicRegistrationDataTest.php

@@ -317,7 +317,7 @@ class PublicRegistrationDataTest extends TestCase
 
         RegistrationFormField::factory()->tagPickerField()->create([
             'event_id' => $event->id,
-            'tag_category' => 'Certificaat',
+            'tag_categories' => ['Certificaat'],
             'is_portal_visible' => true,
             'is_admin_only' => false,
         ]);