Improve display of diffs where only new snapshot exists

Currently being displayed transparently:
https://rerun-io.github.io/kitdiff/?url=https://github.com/emilk/egui/pull/7608

<img width="1069" height="853" alt="Image" src="https://github.com/user-attachments/assets/4c9c21ae-6225-4ac4-92d5-062021f9c189" />

Also we should show if the snapshot was updated / added / removed