CloudMirror: Add x64 support and update for Visual Studio 2026 (#424)

- Update AppxBundlePlatforms from x86 to x86|x64 in wapproj
- Add PlatformToolset v145 conditional for VS 2026 (v18.x)
- Update TargetPlatformVersion to 10.0.26100.0 (installed SDK)
- Update MaxVersionTested in Package.appxmanifest to 10.0.26100.0
- Upgrade CppWinRT NuGet from 2.0.220110.5 to 2.0.250303.1
- Update solution file header to VS 18
- Update README build instructions for x64 and modern VS

Co-authored-by: Kiruphakaran P U <kiu@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>