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.

desktop-client/scripts/cargo-xwin

@@ -1,4 +1,12 @@
 #!/usr/bin/env bash
 set -eu
 
+if ! command -v clang-cl >/dev/null 2>&1; then
+  echo "clang-cl is required for Windows cross-builds on Linux."
+  echo "Install LLVM/Clang toolchain first, for example on Ubuntu:"
+  echo "  sudo apt update"
+  echo "  sudo apt install -y clang lld llvm"
+  exit 1
+fi
+
 exec cargo xwin "$@"