MSIX packaging and code-signing for Windows release artifacts

[OldCrow]

Summary

The Windows build produces an unsigned .exe with no MSIX package. The README marks MSIX packaging as a future item. Without it, distributing a signed release artifact for Windows is not possible.

Action needed

1. MSIX manifest: Create Package.appxmanifest with correct Identity, Properties, Applications, and Capabilities entries for the WinUI 3 app.
2. Assets: Add required logo and splash-screen assets at the sizes required by the Windows App Packaging project.
3. Publisher certificate: Obtain or generate a code-signing certificate. For open-source use, a self-signed cert is sufficient for local testing; a trusted certificate (EV or otherwise) is needed for public distribution.
4. Build integration: Update scripts/build-windows.ps1 to produce a signed .msix package when run with a -Package flag.
5. CI artifact: Add a step to the Windows CI job (.github/workflows/ci.yml) to upload the unsigned MSIX as a workflow artifact on tagged commits.
6. README update: Document the Windows packaging and signing steps in frontend/windows/ewcalc-winui/README.md.

Files

• frontend/windows/ewcalc-winui/ — manifest and assets
• scripts/build-windows.ps1 — packaging step
• .github/workflows/ci.yml — artifact upload
• frontend/windows/ewcalc-winui/README.md