Shopify API Migration Required: REST → GraphQL + OAuth

The `ecommerce_integrations` app currently uses the Shopify REST Admin API (version `2024-01`) with API token authentication. Shopify has deprecated this approach and it is no longer possible to connect new Shopify sites. Existing setups may continue to work only during Shopify's grace period.

### Background

Shopify has made the following breaking changes to their platform:

- The **REST Admin API is officially a legacy API** as of October 1, 2024
- Starting **April 1, 2025**, all new public apps must be built exclusively with the [GraphQL Admin API](https://shopify.dev/docs/api/admin-graphql)
- New API credentials (private app tokens) **can no longer be created** for the old REST approach
- **OAuth** is now the required authentication mechanism for new integrations

Start by locating the REST Admin API version 2024-01 usage and API-token authentication within the ecommerce_integrations app. Read Shopify's linked GraphQL Admin API documentation and define the OAuth flow and migration scope. Done means new Shopify sites can connect through GraphQL and OAuth without relying on the deprecated REST approach.

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