feat: Improve error handling when login fails after signup

[amitsingh05667]

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Problem

When signupAndLogin creates the account but the automatic login fails, the user only sees the login error (“Wrong email or password”).

They don’t know the signup actually succeeded, so they try again and immediately hit “user already exists”. This creates a bad loop.

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What I changed

• Added a clearer error message explaining that signup worked but login failed.
• Added a new signupSucceeded: true flag on the error object.
• Kept the original login error text for debugging.

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