[WIP] Fix: Always assign referral ID for external logins (issue #742)

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original description:

Update the external login authentication flow so that a referral ID is always set for users who log in with external providers (e.g., Google, Microsoft). This prevents REFERRAL_ID from being null for authenticated users by ensuring the referral claim is present.

Steps:

• Inject IReferralService into the ExternalLoginModel (Areas/Identity/Pages/Account/ExternalLogin.cshtml.cs).
• After a successful sign-in (SignInManager.SignInAsync), call referralService.EnsureReferralIdAsync(user) before redirect.
• Test that users logging in with external providers always have a referral ID assigned.

This addresses issue #742.

This pull request was created as a result of the following prompt from Copilot chat.

Update the external login authentication flow so that a referral ID is always set for users who log in with external providers (e.g., Google, Microsoft). This prevents REFERRAL_ID from being null for authenticated users by ensuring the referral claim is present.

Steps:

• Inject IReferralService into the ExternalLoginModel (Areas/Identity/Pages/Account/ExternalLogin.cshtml.cs).
• After a successful sign-in (SignInManager.SignInAsync), call referralService.EnsureReferralIdAsync(user) before redirect.
• Test that users logging in with external providers always have a referral ID assigned.

This addresses issue #742.

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