update to new-format TFA login

update to new-format TFA login

use the initState method to avoid that we override the initial
selected TFA method on every re-build and also to avoid running into
a transitive error when TFA login suceeded, as the handler for that
in the API dart lib will set tfa to null then, causing a null
dereference error if we'd get that in the (re)build method.

Co-authored-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>