docs: update demo.gif (#1451)

## What?

Replaces `public/assets/demo.gif` with a freshly recorded VHS run.

## Why?

Keeps the demo GIF aligned with the latest release so README and docs
reflect current behaviour. Generated automatically by the Update Demo
VHS workflow.

Co-authored-by: floatpanebot <278062430+floatpanebot@users.noreply.github.com>

Author: floatpanebot