[native] derive SIWE url instead of hardcoding it

Summary: we now derive the SIWE url so it works the same regardless if it's a sim/physical device or in prod. linear task

Test Plan: run app, sign out & tap SIWE on sim, then physical device. i also tested ignoring __DEV__ flag to ensure we visit https://comm.app/siwe & it worked.

Reviewers: atul, rohan, ginsu, ashoat

Reviewed By: ashoat

Subscribers: abosh, tomek, ashoat, ginsu, rohan, atul

Differential Revision: https://phab.comm.dev/D5382