chore: use the ShellUI Native mark for the package, demo app and READMEs

[Shewart]

Summary

Applies the ShellUI Native brand mark (two-screens mark from the Shelltech brand set).

• NuGet package icon: assets/icon.png set as PackageIcon in Directory.Build.props, so ShellUI.Native.CLI ships with the mark.
• MAUI demo: app icon redrawn on an ink background with the dots kept inside the Android safe zone; splash redrawn; Color changed from #512BD4 (MAUI purple) to #0A0A0A.
• READMEs: the mark above docs/README.md and the CLI README, as a <picture> that switches to assets/icon-dark.png on dark themes.

Verification

• Release build: 0 warnings, 0 errors
• Tests: 488/488 pass
• dotnet pack of the CLI: the .nupkg contains icon.png
• Android resizer output shows the mark in the generated launcher PNGs

No component or CLI behaviour changes.