Desktop: Remove POSIX dependencies from snapshots and runtime

[priethor]

What

This is the first app-side PR for Windows support. It replaces zip, unzip, and which with JavaScript helpers, adds Windows-aware executable lookup, and makes site extraction and runtime shutdown asynchronous.

The site refresh keeps the existing database, uploads, and wp-config.php. There's no Windows runtime or installer here yet, and this PR doesn't touch Buildkite or signing.

Why

Most of the desktop app still assumes macOS or Linux paths, tools, and process signals. We need to remove those assumptions before the Windows build can start and shut down cleanly.

How

Cortext now checks ZIP entries before extraction and only moves a site into place once the extraction has finished. On Windows, PHP uses php.exe, runs as one hidden worker without POSIX process groups, and falls back to taskkill if it doesn't exit normally.

The app also waits for PHP to stop before quitting or applying an update.

Only one copy runs at a time. Opening Cortext again brings the existing window to the front instead of starting a second app against the same site folder.

Testing Instructions

1. Rebuild snapshot.zip from apps/desktop.
2. Launch Cortext and confirm that an existing workspace opens normally.
3. Make a change, quit Cortext, and reopen it to confirm the change is still there.
4. Close Cortext once while it is loading and once after it has loaded. Wait for it to quit fully, then reopen it and confirm it starts both times without a port 9402 error.
5. With Cortext already open, launch it again from Finder or the dock. Confirm that no second window opens and the existing one comes to the front.