[landing] Fix SIWE after RainbowKit/Wagmi update

Summary:
I'm not sure if this is happening due to the RainbowKit or Wagmi upgrades, but RainbowKit's connectModalOpen is now turning to false a split second before signer is set, which is causing this code to close the modal.

This setTimeout gives us a second to wait and see if signer is set before we decide if the modal closed to the user exiting it or due to a successful wallet auth.

Test Plan: I tested this locally in my dev environment with landing and native both running in dev mode. I tested both X'ing out of the modal (which should trigger this exit condition) and the normal log-in flow (which should NOT trigger)

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6738