Self-update for app and installer. Integrates cleanly with the
worker-log-footer feature that landed on main in parallel — the
shell VM now carries both worker-log state and update-check state,
and MainWindow hosts both the update banner and the footer log line.
Conflict resolved in IslandsShellViewModel.cs: kept nullable property
types from main's test-only parameterless constructor work, and added
the UpdateCheck property exposing the injected service.
0b72c0fb53