[lib] have landing send fid as a string and not a number

Summary:
native expects a string but fid received is a number.

Context: https://linear.app/comm/issue/ENG-7679/type-mismatch-bug-in-farcaster-web-view-reactjs

Test Plan: flow check. Checked on native that receiving correct string fid after connecting my farcaster account

Reviewers: ginsu, atul, ashoat

Reviewed By: ginsu, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11626