docs: add clang/lld/llvm prerequisites for Linux Windows cross-builds

Add apt install instructions for clang, lld, llvm, and nsis to README Ubuntu-to-Windows build section. Add clang-cl availability check to cargo-xwin wrapper script with helpful error message directing users to install LLVM/Clang toolchain when missing.

README.md

@@ -63,6 +63,8 @@ Ubuntu-to-Windows `Setup.exe` build:
 
 ```bash
 cd desktop-client
+sudo apt update
+sudo apt install -y clang lld llvm nsis
 cargo install --locked cargo-xwin
 rustup target add x86_64-pc-windows-msvc
 npm install