Consistency in whether `Auth.Flow` functions take an `Auth.Common.ToBackend -> Cmd frontendMsg` parameter

[SiriusStarr]

Currently, Auth.Flow.init takes a toBackendFn to turn the produced Auth.Common.ToBackend to an arbitrary Cmd msg. Auth.Flow.signInRequested, however, does not, necessitating it always be followed by a Tuple.mapSecond toBackendFn.

It feels as if either both or neither of these should take a toBackendFn parameter (both probably being the more ergonomic option).