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/package.json

@@ -10,7 +10,7 @@
     "tauri:build": "tauri build",
     "helper:windows-x64": "bash ./scripts/build-tunnel-helper.sh x86_64-pc-windows-msvc",
     "helper:macos-arm64": "bash ./scripts/build-tunnel-helper.sh aarch64-apple-darwin",
-    "tauri:build:windows-x64:linux": "chmod +x ./scripts/cargo-xwin && PATH=\"$PWD/scripts:$PATH\" XWIN_ARCH=x86_64 tauri build --runner cargo-xwin --target x86_64-pc-windows-msvc --config src-tauri/tauri.windows.conf.json"
+    "tauri:build:windows-x64:linux": "chmod +x ./scripts/cargo-xwin ./scripts/clang-cl && PATH=\"$PWD/scripts:$PATH\" XWIN_ARCH=x86_64 tauri build --runner cargo-xwin --target x86_64-pc-windows-msvc --config src-tauri/tauri.windows.conf.json"
   },
   "dependencies": {
     "@tauri-apps/api": "2.10.1",

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 "$@"