Add editor management per storefront

## Summary
Add support for assigning editor users to individual storefronts.

## Context
Registration currently creates admin users only. Storefront ownership exists, and products/articles are now scoped to storefronts. We need a later workflow for storefront owners/admins to invite or assign editor users to a specific storefront without giving them full platform admin access.

## Acceptance criteria
- Storefront admins can add an editor to a storefront.
- Editors are scoped to one or more storefronts.
- Editors can manage products/articles only for assigned storefronts.
- Platform admins can view and manage storefront editor assignments.
- Registration remains simple and does not expose role selection to public users.