[A11y] macOS desktop app: VoiceOver navigation jumps the cursor around; sidebar and sessions hard to move through

## Summary

I'm blind and use VoiceOver full time, and I run Claude Code through the macOS desktop app. Two things make it hard to get around with a screen reader.

## Environment

- Claude desktop app for macOS 1.32885.1 (com.anthropic.claudefordesktop)
- Claude Code 2.1.234
- macOS 26.6.1
- VoiceOver

## What happens

1. VoiceOver navigation jumps the cursor. This is the big one. Moving by heading is unpredictable, and using the VoiceOver navigation keys to move through the conversation sends the cursor somewhere I didn't ask for, seemingly at random. I can't read a reply in order or go back to something I half heard without losing my place.

2. Sidebar and sessions. Moving between sessions and projects, and using the sidebar controls, is hard. It's easy to lose track of where I am and how to get back to the conversation.

One thing that works and should stay: VoiceOver says "Claude is finished responding" when a turn ends, and I get a notification if I'm in another app. That's the right idea.

## What would help

- A stable reading order, so VoiceOver's next/previous and heading navigation move through the transcript in sequence and stay where I put the cursor.
- Each message as a heading that says who said it, so the rotor can move between turns.
- Labelled sidebar controls, and a keyboard shortcut back to the conversation.

Related: #70425 (screen reader usability for Claude Code generally) and #76252 (composer re-announcing under VoiceOver on desktop).

I'll test builds or answer questions if that's useful.

Reproduce the VoiceOver behavior in the macOS desktop app using the reported versions, focusing on transcript navigation, message headings, sidebar controls, and returning to the conversation. Done means VoiceOver reads the transcript in stable order, can move between speaker-labelled turns, and keeps sidebar navigation understandable without disrupting the current position.

索引モデルが issue の本文から書いたものです。