Updater: real-layout apply with rollback, in-app update + restart

- Strip the tarball's top-level folder; stage inside the install root
  (same filesystem) and swap with full rollback; keep file modes.
- Semver prerelease ordering (1.2.3-rc1 < 1.2.3).
- UpdateController: background check, download with progress, apply,
  restart; required-update wording for newer protocols; Check now.
- update.sh/update.bat in the bundle; Windows apply path (untested).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/update/bundle/update.sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+# Update this install in place (same as clicking Update in the app).
+cd "$(dirname "$0")" && exec ./watch_together --upgrade "$@"