fix: use cargo-xwin wrapper script to resolve Tauri runner compatibility

Add cargo-xwin wrapper script that invokes `cargo xwin` subcommand and update Linux Windows build script to use wrapper as --runner instead of cargo-xwin binary name directly. Add chmod +x to ensure wrapper is executable before build.

desktop-client/scripts/cargo-xwin

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+set -eu
+
+exec cargo xwin "$@"