Display byte units in text list/get output

[nanookclaw]

Updates the text render paths for tart list and tart get so their Disk and Size values are shown with ByteCountFormatter human-readable units, while preserving the existing field and column names.

The JSON paths keep their previous machine-readable behavior for compatibility: tart list --format json still emits integer GB values for Disk and Size, and tart get --format json still emits integer GB for Disk plus the existing three-decimal GB string for Size.

Focused tests cover both compatibility shapes and the human-readable text projections.

Fixes #820

Testing

• git diff --check
• Not run: swift test --filter 'GetTests|ListTests' (swift is not installed in this environment)

[edi-oai]

Sorry, perhaps I'm missing the point, but it feels like we can do this much simpler and minimize the overall churn.

What do you think about #1301 as an alternative implementation?