Modern English desktop and six-platform preview releases

[fmarslan]

Scope

• Modern English native desktop UI with default application-data startup.
• Native Windows, macOS and Linux AMD64/ARM64 CI tests and release builds.
• Six archive packaging, checksum verification and preview publishing after main builds pass.

Validation

• Local container: 22 unit tests and one CLI integration test passed.
• Rust formatting and Clippy passed; packaging regression test and actionlint passed.
• All nine uploaded files match the local Git blob hashes.
• Native GitHub matrix results are pending.

Limitations

This is a preview, not completion of roadmap phases 6–13. Cloud/browser/Git-backed integrations remain pending. Packages are unsigned and macOS is not notarized.