pkg/scanner: replace go-bytesize with local formatter

[thaJeztah]

The scanner only uses go-bytesize to format file sizes for threshold
violations. Replace the dependency with a small local helper that formats
sizes using powers of 1024 and two decimal places.

This keeps the required behavior while avoiding a dependency on the broader
parsing and formatting functionality provided by go-bytesize.

[thaJeztah]

@defangdevs @lionello PTAL: 🤗

[defangdevs]

Thanks for the PR! The Test job failure here isn't caused by this change — main itself has been failing the same way since #5 added CI (tracked in #7, fix already up in #6 pending review/merge). Your bytesize-formatting diff looks unrelated to any of those test fixtures. Once #6 merges you should be able to rebase and get a clean run; will ping here if that lands.