MSIX: replace placeholder Store logo assets with correctly-sized PNGs

[plusmobileapps]

Context

The Microsoft Store (MSIX) packaging added in #229 currently reuses the app icon (client/composeApp/src/jvmMain/resources/app-icon.png) for all three required Store logo assets, generated at build time in the Package MSIX (Windows) step of .github/workflows/desktop-release.yml:

• Assets/StoreLogo.png
• Assets/Square150x150Logo.png
• Assets/Square44x44Logo.png

This lets makeappx produce a valid MSIX, but a single unscaled icon will not pass Microsoft Store certification, which expects assets at their specified dimensions (and ideally the full scaled asset set).

Task

• Produce correctly-sized PNG assets for the MSIX visual elements (at minimum the three above; ideally the full set of scale/target-size variants).
• Commit them under packaging/windows/Assets/ (or generate them at build time from a high-res source).
• Update the Package MSIX (Windows) step to copy the real assets instead of duplicating app-icon.png.
• Confirm the manifest's <Logo> and uap:VisualElements references resolve against the new asset names.

References

• Manifest: packaging/windows/AppxManifest.xml
• Workflow step: .github/workflows/desktop-release.yml → Package MSIX (Windows)
• Docs: docs/deployment.md → "Windows: Microsoft Store (MSIX)"
• App icon assets for MSIX

Follow-up from #229.