Fix native Windows desktop builds and verify the installer in CI

[Xenox258]

Native Windows desktop builds currently fail on the POSIX environment assignment, CRLF version checks, and paths containing spaces. This change makes npm run desktop:build work on Windows and fixes the packaged server's startup and shutdown.

• Use a cross-platform build entry point and accept CRLF in the version check.
• Bundle Node directly as the Windows sidecar, use the inherited Windows PATH, and launch the server relative to its working directory. This avoids a forwarding child process and a duplicate runtime.
• Detect parent termination on Windows and cover it with a process lifecycle regression test.
• Add a Windows workflow that installs the locked dependencies, builds in a path containing spaces, silently installs the NSIS package to another path containing spaces, checks HTTP readiness and normal shutdown, and uploads the installer plus SHA-256 checksum.

Validation: native Windows x64 build passed locally. The Windows CI run passed the build, parent-termination test, installation, HTTP readiness check, and normal shutdown check. No provider keys or paid generations are needed for these checks. macOS and Linux builds were not run.

Windows build: download the installer and checksum. This is an unsigned x64 NSIS installer built from ab3298642bacf3490a47b5c514e9ffd85e976d80. The CI artifact expires on 25 September 2026; the workflow can regenerate it. The installer can be attached to the release after maintainer review.

DESKTOP.md documents the Windows prerequisites, verification commands, and Tauri's local-tools override for machines with a restricted or encrypted user cache.