chore(nuget): optimize package icon to 512x512

The gallery icon was 1692x1692 (150 KB) — above NuGet's ~1024px practical max.
Downscale the same FSH mark to 512x512 (53 KB), preserving transparency. Embedded
at the root of both the CLI and template packages via PackageIcon.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: iammukeshm