App: run update jobs in the background and swap the release in after closing (#231)

The desktop app checks, downloads, verifies and stages on its own threads for the window, and
once the window has closed (autosave closed) swaps a staged release in and starts it. Staging
folders an earlier update left are removed at start-up. Automatic checks stay off under
--control automation and CI unless FIELDCRAFT_UPDATE_HOST names a test server.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

Cargo.lock

@@ -1278,6 +1278,7 @@ dependencies = [
  "fieldcraft-engine",
  "fieldcraft-io",
  "fieldcraft-ui-egui",
+ "fieldcraft-update",
  "getrandom 0.3.4",
  "log",
  "rfd",