docs(desktop): Windows README says "no extra dependencies" but MSVC linker is required

When running moon run desktop:dev on Windows without Visual Studio / Build Tools installed, the build fails with:

`error: linker `link.exe` not found`
`note: the msvc targets depend on the msvc linker but `link.exe` was not found`

apps/desktop/README.md currently states under "Platform requirements":

Windows: no extra dependencies (Win32 + DirectWrite).

This isn't accurate - the MSVC linker (from Visual Studio Build Tools, "Desktop development with C++" workload) is required to compile Rust crates on Windows. Installing it via:

`winget install --id Microsoft.VisualStudio.2022.BuildTools --override "--quiet --wait --add Microsoft.VisualStudio.Workload.VCTools --includeRecommended"`

resolves the build.

I have a one-line doc fix ready on my fork (branch docs/desktop-windows-msvc-prereq) clarifying this requirement, happy to open a PR if that's welcome given the current contribution policy.