[web] Proactively fetch siweNonce once user has indicated they intend to SIWE

Summary: Context: https://linear.app/comm/issue/ENG-2741/[web]-proactively-fetch-nonce-once-user-has-indicated-they-intend-to

Test Plan: Works as expected:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6340