Support Apple auth

[gnprice]

AKA "Sign in with Apple". This will be a requirement for launch, because Apple makes it a requirement for the App Store. (That doesn't affect the beta, which is in TestFlight and not in the App Store.)

Upstream docs here:
https://developer.apple.com/sign-in-with-apple/
but I hope we can complete this feature without (re-)reading those in any detail — instead we can crib from the work we did back in 2020, when we read up on this closely in order to implement it the first time.

I doubt any significant number of people are actually using this feature, so there's probably no need for it in the beta period. But we'll see if we hear feedback asking for it. Indeed I don't think we've heard from anyone wanting to use it. But now that launch time is getting close, we'll need to do this sometime soon.

Implementation

Because this issue is closely tied to our interaction with Apple, it will need to be done by a member of the core team.

There are two UX flows for Sign in with Apple:

• a web-based flow (this issue);
• a native iOS flow: Apple auth via native iOS flow #1442.

For this issue, we'll concentrate on the web-based flow, which is available on iOS and on Android. The UX this provides is the same kind of UX as when people log in via GitHub, or Google, or any other form of SSO.

I believe the web flow may actually already work, as part of our support for web-based auth in general. In that case the key task remaining for this issue is to test the web flow manually to confirm that it works smoothly.

Testing

An important part of the work on this issue will be to test it end to end. The only practical way to do that is manually: fire up the app with your draft implementation of the feature, and try using it.

To test the implementation that's in the published beta app, try running the app and logging in on chat.zulip.org.

For instructions on how to test Sign in with Apple in a development build of the app, see zulip/zulip-mobile@c19cbe2, or perhaps more readably:
https://github.com/zulip/zulip-mobile/blob/main/docs/howto/ios-tips.md#sign-in-with-apple
See also this 2025 thread which has some updates to those instructions:
#mobile-dev-help > Enable "Sign in with Apple" for Flutter App

Related issues

• From when we originally implemented this feature in the server and the legacy mobile app, in 2020 when Apple's mandate of it took effect:
  Support "Sign In with Apple" auth zulip#14168
  Support "Sign In with Apple" auth [iOS] zulip-mobile#3964
  Those have a lot of links to Apple docs and other background information. (But again, I hope we can skip reading or re-reading most of that — instead borrow from the work we did in 2020.)

• A follow-up issue: Apple auth via native iOS flow #1442

[gnprice]

I've just edited this issue to split it in two:

• This issue, which remains a launch-blocker priority issue, is now focused on making sure "Sign in with Apple" works smoothly via its web-based flow.
• The iOS native flow is now a follow-up issue, which I believe we can save for a later milestone: Apple auth via native iOS flow #1442.