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/usr/share/applications/appimagetool.desktop

@@ -0,0 +1,9 @@
+[Desktop Entry]
+Type=Application
+Name=appimagetool
+Exec=appimagetool
+Comment=Tool to generate AppImages from AppDirs
+Icon=appimagetool
+Categories=Development;
+Terminal=true
+