Velopack packaging on all platforms with in-app auto-updates

- macOS: .pkg installer replaces the DMG; Linux: .AppImage replaces the ZIP
- winmain runs velopack.App() first (proper hook handling) and checks for
  updates in the background; downloads are applied on next launch
- release.py uploads the vpk update feed (releases.<channel>.json, nupkgs)
  so GiteaSource finds updates on the latest release
- guibuild.py bootstraps a .NET runtime into build/dotnet when the runner
  host lacks one

pyproject.toml

@@ -52,6 +52,7 @@ parse-torrent-title = { git = "https://github.com/platelminto/parse-torrent-titl
 gui = [
     "pywebview[qt]>=6.2.1; platform_system != 'Windows'",
     "pywebview>=6.2.1; platform_system == 'Windows'",
+    "velopack>=1.2",
     "qtpy>=2.4.1; platform_system == 'Darwin'",
     "PyQt5>=5.15.11; platform_system == 'Darwin'",
     # pywebview[qt] no longer pulls this in; the macOS Qt backend needs it