fix: add clang-cl wrapper script and update Windows build to ensure clang availability

Add clang-cl wrapper script that invokes clang in MSVC-compatible mode with availability check and helpful error message. Update Linux Windows build script to make clang-cl wrapper executable and include scripts directory in PATH for both cargo-xwin and clang-cl resolution.

desktop-client/scripts/clang-cl

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+set -eu
+
+if ! command -v clang >/dev/null 2>&1; then
+  echo "clang is required for Windows cross-builds on Linux."
+  echo "Install it with:"
+  echo "  sudo apt update"
+  echo "  sudo apt install -y clang"
+  exit 1
+fi
+
+exec clang --driver-mode=cl "$@"