Update all Windows build configurations, scripts, and documentation from i686-pc-windows-msvc (x86) to x86_64-pc-windows-msvc (x64). Update npm scripts, build-tunnel-helper.sh target validation, bundled helper paths, and tunnel manager strategy references. Add XWIN_ARCH=x86_64 environment variable to Linux cross-build command and --xwin-arch flag to cargo xwin invocation.
Change cargo-xwin detection from `command -v cargo-xwin` to `cargo xwin --version` to properly verify cargo subcommand availability on Linux systems cross-compiling to Windows targets.
Add Ubuntu-to-Windows build workflow using cargo-xwin for i686-pc-windows-msvc target. Update build-tunnel-helper.sh with host OS detection, target validation, and cargo-xwin support for Linux cross-compilation. Add tauri.windows.conf.json to configure NSIS-only bundle output and npm script for Linux-based Windows builds. Update client-platforms.md to document cross-compilation support and clarify that MSI packaging still requires Windows environment.
