fix(setup): exit gracefully when initial setup is canceled

[mstrhakr]

Summary

Treat dismissing the initial setup dialog as normal cancellation rather than a startup failure.

• Return exit code 0 instead of 1 when Cancel or window close dismisses the setup dialog.
• Keep genuine failure handling and the loader's nonzero-exit checks unchanged.
• Add subprocess regression coverage for both false/null dialog results, asserting a quiet successful exit before setup continues.
• Document cancellation behavior in the README.

Validation

• Rebased onto the latest origin/main (already up to date).
• PowerShell 7: all 47 utility Pester tests passed after rebase.
• Regression cases execute only the extracted dialog-decision block, not machine configuration.
• Diff whitespace checks passed; working tree is clean.
• Windows PowerShell 5.1 remains covered by repository CI; local machine policy prevents unsigned script execution in that runtime.

Release

This changes src/scans.ps1, the versioned Windows utility. Publish a new Scans patch release after merge to deliver the fix through the stable loader. A site-only refresh does not update existing release assets.