I'm building an app for iPhone and this is terrible user experience. That view is not responsive to the screen size and even zooming out is disabled.

That really needs to be fixed before it can be handed to users.

The sign in page on the other hand is done properly:

Agreed!

I'll help you fix this:

1. in that page, the element with id="oauthWrapper" has style="width:32rem". It translates to 512px, more than the logical pixel width of an iPhone 15 of 393px. 
   Replace that with style="max-width:32rem" and we're good.
2. When "All Workspaces" is selected, the message "All current and future bases in all current and future workspaces" has the class "truncate". Remove it to let it wrap.

See how good it looks now: