refactor: replace formatBytes with humanize.IBytes

[deevus]

Summary

• Replace hand-rolled formatBytes function with humanize.IBytes() from dustin/go-humanize, which was already an indirect dependency
• Delete TestFormatBytes (now testing library code)
• Net -36 lines

Details

The 12-line formatBytes in cmd/root.go reimplemented humanize.IBytes() identically. Since go-humanize was already in the dependency graph (pulled in by truenas-go), this is a pure code reduction with no new dependencies — just promotes it from indirect to direct.

Test plan

• go build compiles
• go test ./... all pass
• go vet ./... clean

[github-actions]

⬇️ Go test coverage decreased from 27.1% to 26.5% compared to a683de1

⚠️ 2 of 8 packages have zero coverage.

• github.com/deevus/pixels
• github.com/deevus/pixels/internal/cache

Updated Package Coverages:

# Package Name                 |  Prior |    New
- github.com/deevus/pixels/cmd |   8.1% |   6.8%

View coverage for all packages

# Package Name                              | Coverage
- github.com/deevus/pixels                  |     0.0%
+ github.com/deevus/pixels/cmd              |     6.8%
- github.com/deevus/pixels/internal/cache   |     0.0%
+ github.com/deevus/pixels/internal/config  |    85.2%
+ github.com/deevus/pixels/internal/dataset |   100.0%
+ github.com/deevus/pixels/internal/egress  |    93.5%
+ github.com/deevus/pixels/internal/ssh     |    24.1%
+ github.com/deevus/pixels/internal/truenas |    68.5%