Review screen-reader usability across the app

[gnprice]

This is a followup to #535, an expanded version to be comprehensive across the app's functionality. This issue is effectively the zulip-flutter successor to the open zulip-mobile issue zulip/zulip-mobile#4394 .

For this issue, the task is to learn to use the screen reader provided by your device's operating system (VoiceOver on iOS, TalkBack on Android), then try to use it with all kinds of features in the app, and see what doesn't work or doesn't work as well as it could.

For everything we find that can be improved, we'll file and triage issues so we can fix them. Use the a-a11y label so that it's easy to find the list of them.

How to approach this issue

See my discussion in zulip/zulip-mobile#4394 about how to start learning to use these systems and to evaluate the app with them.

As part of #535 we'll have covered the essentials of the app's functionality before we start on this followup issue. Other feature areas to cover include:

• reactions
• all kinds of message content features (including interactions like links, images, and spoilers, and Zulip "widgets" like polls)
• the lightbox
• action sheets and their actions
• compose typeaheads
• and everything else in the app.

As part of this issue we'll also be evaluating to higher standards of UX quality than for #535. See zulip/zulip-mobile#4394 for examples of kinds of glitches that can be present, and that we should find and fix:

• properly announcing what page you're on, when you enter a list of items, etc.
• all buttons, even those less commonly used, should work and should have captions
• nothing should be selectable that doesn't actually do anything or give any useful information
• proper conventions for navigating between messages, announcing message sender/time/etc., and so on
• and anything else that comes up that seems like it'd get in the way of using the app's features successfully or efficiently.

For the issues we find as part of this review, generally the fixes will involve Flutter's Semantics widget and related parts of the Flutter API, to provide the appropriate semantic information for screen readers (like VoiceOver and TalkBack) and other assistive technology to use. But this issue is only about reviewing the app to find those issues; we'll use their own respective issue threads to track actually fixing them.