batches: improve screen reader experience with server-side workflow

Created by: courier-new

Okay so technically this closes #34074 (closed), #34075 (closed), #34076 (closed), #34077 (closed), and #34087 (closed). But we have some serious follow-up to do on the screen reader experience front with SSBC.

I spent the better half of today trying to wrestle with screenReaderAnnounce and live content alerts, gracefully transfer focus when previously interactive items become un-interactive or are removed from the DOM entirely, debug problems with multiple elements fighting for screen reader attention at the same time, and figure out how on earth to even begin to coherently convey everything that's changing all the time on the execution screen. I tore most of it out at the end of the day because it either didn't work right, was way too hacky to commit, was way too fragile and difficult to test, or all of the above (most of the time it was all of the above). 😞 The new SSBC UI is fabulous and expressive and remarkably detailed, but that complexity comes with an accessibility cost. I think it's in our best interest to reprioritize investing time into this until after the UI and SSBC workflows have stabilized after beta feedback and we have better application-wide guidelines on accessibility design for dynamic UIs. I've filed https://github.com/sourcegraph/sourcegraph/issues/38789 in our backlog to track this.

So anyway, what remains here is the bare minimum: a few changes here and there that seemed to be net improvements that work fairly consistently (screenReaderAnnounce's behavior still eludes me sometimes...).

Test plan

Lots of VoiceOver testing. That's the best I can do right now.

App preview:

• Web
• Storybook

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