Semantics for loading screen/indicator

[gnprice]

This was found in https://github.com/zulip/zulip-flutter/issues/535#issuecomment-2973276995 when investigating the app's behavior with a screen reader:

The loading screen/loading indicator cannot get focused. There should at least be some way to tell if the page is loading.

We should fix this.

A required part of any PR to address this issue will be to try it out with a screen reader and confirm how it behaves. See the description of #535 for instructions.