The current text is a bit wordy and puts the actually important
information at the end:

<img width="607" height="120" alt="image"
src="https://github.com/user-attachments/assets/26f998dd-1801-4e6c-a40c-acb58f8f3efc"
/>

The button already shows as disabled, so the text just needs to explain
why.

I don't think this affects accessibility, because a screen reader should
be able to tell that the button is disabled.

Self-Review Checklist:

- [ ] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [ ] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [ ] Performance impact has been considered and is acceptable

Release Notes:

- N/A