Add website dark mode and unblock Store release

[Xueyang-Song]

Summary

• removes the obsolete WinUI Store workflow migration gate
• adds system-aware website light/dark theme tokens with a header switch
• swaps landing page product screenshots between light and dark variants
• adds dark screenshot variants for the landing showcase images

Validation

• dotnet build .\JitHub.slnx -c Debug -p:Platform=x64
• dotnet publish .\JitHub.Web\JitHub.Web.csproj -c Release -o .\artifacts\web-publish-smoke-dark
• browser automation captured light, dark, and clicked-switch screenshots under artifacts/screenshots/web