This addresses ENG-7391. Instead of failing at the registration step later, we want to check if the Farcaster account is already linked at the ConnectFarcaster screen. We think it's a better user experience to "fail fast".
I considered implementing this at the FarcasterWebView layer, so that FarcasterAccountSettings and ConnectFarcasterBottomSheet could use it. But those screens will call the identity service immediately, so there's no point "pre-checking" with the identity service.