login: Avoid showing email-password option when disabled by server

[rajveermalviya]

This will require checking the authentication_methods.password or email_auth_enabled field in the server-settings API response. If this field is disabled, the login page should only display the enabled external authentication method buttons.

Docs: https://zulip.com/api/get-server-settings#response

(Noticed while logging in xi—zulipchat—com)

[pateldhruv0248]

Hello, I want to contribute to this issue.
Please note, this would be my first issue on Zulip organization. Thanks.

[gnprice]

@pateldhruv0248 Welcome. See our guide to picking an issue to work on:
https://zulip.readthedocs.io/en/latest/contributing/contributing.html#picking-an-issue-to-work-on

[pateldhruv0248]

Hello,
I have created this PR in my fork as a proposed solution:
pateldhruv0248#1

[gnprice]

Cool. Please send the PR to this repo, and then we can review it.

Before we can review it, the PR will need tests and to have a commit message following our style. See docs:
https://github.com/zulip/zulip-flutter#submitting-a-pull-request
(You can send the PR as a draft when it's not yet ready for review.)

[chrisbobbe]

This was requested on CZO: #mobile > Login screen when EmailAuthBackend is disabled @ 💬