bug: delays in indexing times can cause keys to get lost when creating new signer

[stephancill]

A brand new signer keypair is generated on mounting of the sign in with warpcast/ethereum component. This means that if the modal closes for some reason while waiting for the new signer transaction to be confirmed/the warpcast API to respond, the key will be lost.

This key should be persisted with an expiry time and loaded while the expiry time is not in the past. On mount, the component should check if the key in localstorage has been indexed, and if so, should add it to the list of signers and skip the step of registering it as a new signer.