Preserve environment when spawning sshfs

[Evesain]

Summary

Preserve the existing Windows process environment when launching sshfs.exe, while still prepending the SSHFS-Win binary directory to PATH.

Root cause

ProcessHandlerWin currently spawns sshfs.exe with an environment containing only:

env: {
  PATH: dirname(this.settings.sshfsBinary)
}

On current Windows setups this can break SSHFS-Win/Cygwin process startup before it reaches the remote SSH server. In my case SSHFS-Win Manager stayed stuck in connecting, while direct SSH still worked. Reproducing the Manager launch command produced:

failed to create socket pair: Permission denied

Keeping the normal Windows environment and prepending SSHFS-Win's directory allows sshfs.exe to start normally while still resolving the bundled SSHFS-Win binaries first.

Validation

• Reproduced the failure locally with the existing stripped environment.
• Verified the fixed launch behavior by patching the installed app bundle and successfully mounting multiple SSHFS-Win Manager entries.
• Ran node --check src/renderer/ProcessHandlerWin.js.
• Ran git diff --check.

npm ci was not runnable on the current upstream checkout because package.json and package-lock.json are already out of sync for several dependencies; this PR intentionally avoids lockfile churn.

[Evesain]

@evsar3 @lancetekk @BRTPOB could this be looked into and implemented?