fix(desktop): make the Tauri shell work on Windows (#13632)

- Defer updater installation to the user-initiated restart on Windows:
  install() launches the NSIS installer and exits the process immediately,
  so the background cycle now downloads only and stages the bytes, and
  restart_to_apply_update installs them after stopping the sidecar.
- Spawn child processes (sidecar, git, cmd /C start) with CREATE_NO_WINDOW
  so the GUI-subsystem app doesn't pop visible console windows.
- Fall back to USERPROFILE when HOME is unset resolving the MCP settings
  path, matching the sidecar's homedir().
- Reap the sidecar after the Windows hard-kill so its exe file lock is
  released before the NSIS installer replaces it.

Co-authored-by: Saoud Rizwan <saoudrizwan@users.noreply.github.com>