Fix NSIS CI step: use -D defines (makensis on Linux rejects /D as scripts); AppStream metadata hardened (homepage, developer, lowercase id, filename match)

- AppImage validation now passes appstreamcli --pedantic locally; builds with
  metainfo named net.stepline.game.appdata.xml + desktop in usr/share/applications

squashfs-root/AppRun

@@ -0,0 +1,10 @@
+#! /bin/sh
+
+set -e
+
+this_dir="$(dirname -- "$(readlink -f -- "$0")")"
+
+# make appimagetool prefer the bundled mksquashfs
+export PATH="$this_dir"/usr/bin:"$PATH"
+
+exec "$this_dir"/usr/bin/appimagetool "$@"