feat: add site disk size to sites list

[jpollock]

Summary

• Add --size / -s flag to lwp sites list command to show disk usage for each site
• Size includes both site directory (~/Local Sites/<name>/) and MySQL runtime data (~/Library/Application Support/Local/run/<id>/)
• Uses native OS commands for efficient, cross-platform calculation:
  • macOS/Linux: du -sk
  • Windows: PowerShell Get-ChildItem | Measure-Object
• Shows total size at bottom when using --size flag
• JSON output includes size (formatted) and sizeBytes (raw bytes) fields

Closes #4

Test plan

• lwp sites list works unchanged (no size column)
• lwp sites list --size shows Size column with formatted sizes
• lwp sites list -s shorthand works
• lwp --json sites list --size includes size and sizeBytes fields
• Total size shown at bottom (e.g., "Total: 50 site(s), 28.96 GB")
• Unit tests for formatBytes() function (15 tests)
• All 121 tests pass

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