[web] Add <hr /> in LogInForm to separate out "password" and SIWE options

Summary:
Add a <hr /> to separate out "password" and SIWE auth options.

These designs are not final, just matching what's on native as closely as possible for now.

Depends on D6218

Test Plan:
Looks as expected:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6219