Prompt to replace existing passkey instead of creating new item

When I create a new passkey for a website where a passkey already exists in my login item, 1Password creates a completely new item instead of updating the existing one.

Steps to reproduce:

1. Have a login item with a passkey stored for a website
2. Go to that website and create a new passkey (e.g. to replace an expired or revoked one)
3. 1Password creates a new separate item instead of offering to replace the existing passkey

Expected behavior: 1Password should detect the existing passkey for that domain and prompt: "A passkey already exists for this site. Do you want to replace it?"

Current behavior: 1Password silently creates a new item, leaving the user with duplicate entries and confusion about which passkey is valid.

This would be a significant UX improvement for users who need to rotate or replace passkeys.