feat: Integrate referral service in ExternalLoginModel for user referral ID management

[BenjaminMichaelis]

Fixes #742

[Copilot]

Pull Request Overview

Adds a referral‐ID management step to the external login flow by injecting a referral service and calling it when a user signs in via an external provider.

• Introduce IReferralService and configure it in the page model constructor.
• In OnGetCallbackAsync, retrieve the user by external login and invoke EnsureReferralIdAsync.

Comments suppressed due to low confidence (1)

EssentialCSharp.Web/Areas/Identity/Pages/Account/ExternalLogin.cshtml.cs:24

• The injected referralService parameter is never stored in a field, so calls to referralService in OnGetCallbackAsync will not compile. Add a private readonly IReferralService _referralService field and assign it in the constructor (e.g., _referralService = referralService).

    IReferralService referralService) : PageModel