feat: add "Answer Call" button

[WofWca]

Closes #28.

[adbenitez]

at first I wanted to have different pages/components for "incoming call offer" vs in-call vs "pre-videocall start" to reduce complexity, but the logic at App.tsx is already so interconnected and with the complex way of initializing the CallManager that I gave up, also they share components/buttons so maybe lets go for this way of all-in-one single component/page

[adbenitez]

at first this looks a bit not so great to me, I just checked Telegram and this is how incoming videocalls look like:

and this is how an audio-only incoming call looks like:

I think it makes sense to separate the answer + reject from the other options

[WofWca]

Yes, the design is inspired by Telegram.
I suggest to re-do design in follow-up MRs. What we have in this MR now is simple and good enough for a start IMO.

[adbenitez]

Yes, the design is inspired by Telegram. I suggest to re-do design in follow-up MRs. What we have in this MR now is simple and good enough for a start IMO.

sounds good, we can always iterate and improve in another PR, this PR is a draft, is something missing? (besides CI failing)

(EDIT: ah also the readme docs are missing)

[WofWca]

Also regarding the design, see how their desktop version looks:

[WofWca]

I have tested this on Desktop. Works great. See this branch https://github.com/deltachat/deltachat-desktop/tree/wofwca/calls-in-app-accept-user-prompt (you can replace the calls-webapp/index.html manually.

[adbenitez]

great, thanks a lot for pushing this forward! 🙏