[playground] Change auth message for security considerations

[leoguillaume]

Context - why is this issue relevant?

The playground currently exposes different login error messages (invalid password / user not found).
This allows user enumeration and does not follow authentication security best practices.

Objective – what is the aim of this issue?

Replace login error messages with a single generic message that does not reveal whether a user exists or if the password is incorrect.

Expected outcomes – what do we expect at the end of this issue (concrete outcomes)?

Outcomes

• All authentication failures return the same generic error message.
• The frontend no longer exposes whether a user exists.

Acceptance criteria

• Invalid username and invalid password return the same error message.
• No authentication error message reveals user existence.
• Existing tests are updated or added to cover this behavior.