Change icon to use Purr Data branding

The icon for this Flatpak is the Pure Data icon. However we bundle the Purr Data fork, so the icon should reflect this.

Currently there are only small pixmap graphics available for PurrData. Jonathan Wilkes maybe able to provide PurrData graphics in SVG format. See

Inspect the Flatpak metadata and current icon asset to confirm where the Pure Data branding is configured. Obtain or identify a Purr Data SVG replacement, update the icon reference, and verify that the built Flatpak displays the Purr Data branding.

Written by the indexing model from the issue text.