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

apps/fieldcraft/Cargo.toml

@@ -13,6 +13,7 @@ workspace = true
 fieldcraft-engine = { workspace = true }
 fieldcraft-ui-egui = { workspace = true }
 fieldcraft-io = { workspace = true }
+fieldcraft-update = { workspace = true }
 rfd = { workspace = true }
 eframe = { workspace = true, features = ["default_fonts", "wgpu", "accesskit", "wayland", "x11"] }
 egui = { workspace = true }