Desktop: Build the snapshot on Windows

[priethor]

What

Follow-up to #404.

This gets the desktop snapshot building on Windows. The builder no longer assembles wp-cli commands as shell strings, so paths with spaces and quoted seed values are not mangled. A Windows CI job now runs the desktop unit tests and builds snapshot.zip.

The Windows job also caught a POSIX-only assumption in one runtime test, which now explicitly exercises the Linux process path.

Cortext still does not launch on Windows. That needs the bundled PHP runtime.

Why

#404 made the app itself portable, but the snapshot builder still relied on a POSIX shell. This needs to work before we can package Cortext for Windows.

Testing Instructions

1. On Windows, with Node.js, PHP 8.1+, Composer and pnpm installed, check out the repo in a path containing spaces and build the snapshot from apps/desktop. Confirm that snapshot.zip is created.
2. On macOS, set CORTEXT_DESKTOP_EXTRA_SEED_COMMANDS to post create --post_type=page --post_title="Hello there" --post_status=publish, build the snapshot, then launch Cortext with a fresh --user-data-dir. Open the new page and confirm that its title is "Hello there", not "Hello".