Add a custom About window and prepare v0.2.1

[Dezoff-max]

Summary

• replace the sparse system About panel with a centered singleton SwiftUI window
• show live version/build metadata, the local-first safety model, and GitHub, Releases, and MIT License links
• update About content immediately for Russian/English and light/dark appearance changes
• bump the app to version 0.2.1 (build 3)

Verification

• plutil -lint CleanMac/en.lproj/Localizable.strings CleanMac/ru.lproj/Localizable.strings
• swift test --package-path CleanMacCore (15 tests)
• ./script/build_and_run.sh --verify
• live UI review in Russian/light and English/dark, including singleton Window-menu verification
• ./script/package_release.sh
• SHA-256 validation plus strict codesign verification from a fresh ZIP extraction
• independent diff review with no findings

Distribution note

This machine has no Developer ID identity configured. The release workflow therefore produces an ad-hoc-signed, non-notarized arm64 archive and must continue to describe it as unsigned.