Sign-In With Ethereum, done wrong: three replay bugs to avoid

Replace this with the real walkthrough. Suggested outline:

1. Nonce reuse — why a nonce must be single-use and short-lived.
2. Missing domain-binding — how a phishing site can replay a valid signature against your app if the signed message doesn't pin the domain.
3. Account-linking collisions — what happens if two users try to link the same wallet, and why silently merging accounts is the wrong answer.