Login Fails for All Emails Except Demo Customer Email

Describe the bug
In the customer app, users are unable to log in using any email address other than the pre-configured demo customer email. Attempts to log in with a valid registered email result in an error or no response, effectively locking out real users.

To Reproduce
Steps to reproduce the behavior:

Open the customer app
Go to the email login screen
Enter any email other than the demo email (e.g., testuser@gmail.com)
Enter a valid password and attempt to log in
Observe that the login fails with an error
Expected behavior
Users should be able to log in with any valid, registered email address, not just the demo account.

Screenshots

XRecorder_20250619_04.mp4
Smartphone (please complete the following information):

Device: Samsung A15

OS: Android 14

Additional context
This issue may be caused by hardcoded logic tied to the demo account, authentication configuration errors, or backend user validation bugs. It critically affects usability and needs to be resolved to enable access for real customers.

Start at the customer app's email login screen and reproduce the difference between the demo email and a valid registered email on Android 14. Trace the authentication flow and check the demo-account logic, authentication configuration, and backend user validation described in the issue. Done means any valid registered customer can log in successfully, while the demo account continues to work.

Written by the indexing model from the issue text.