docs: add screenshots or a short demo clip to the README

[Hollujay]

Summary

This is a visual tool whose entire value is what the diff looks like, and the
README contains no image of it. A reader currently has to clone and run the app
to find out whether it's worth cloning and running.

Acceptance Criteria

• Add at least one screenshot of a real storage diff to the README
• Use a real contract and a real call, not a mockup
• Keep the image reasonably sized so the README stays quick to load
• Place it near the top, after the description

Tech Stack

Markdown