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

@@ -19,7 +19,7 @@ final class RegistrationFieldTemplateResource extends JsonResource
             'field_type' => $this->field_type->value,
             'options' => $this->options,
             'normalized_options' => $this->normalized_options,
-            'tag_category' => $this->tag_category,
+            'tag_categories' => $this->tag_categories,
             'is_required' => $this->is_required,
             'is_filterable' => $this->is_filterable,
             'is_portal_visible' => $this->is_portal_visible,