When deleting a sandbox with dependents, provide a warning

When merging a sandbox with dependent sandboxes, we show a nice warning:

<img width="1140" height="556" alt="Image" src="https://github.com/user-attachments/assets/cb0723d1-7e7f-4caa-8e11-ecad1a91f808" />

But we don't do this on delete:

<img width="470" height="391" alt="Image" src="https://github.com/user-attachments/assets/bd0843e2-5079-421e-bd5f-45724e0fce83" />