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
+

squashfs-root/usr/share/metainfo/appimagetool.appdata.xml

@@ -0,0 +1,31 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop">
+  <id>appimagetool.desktop</id>
+  <metadata_license>MIT</metadata_license>
+  <project_license>MIT</project_license>
+  <name>appimagetool</name>
+  <summary>Create AppImage files</summary>
+  <description>
+    <p>
+      appimagetool is a tool that lets you generate AppImage files. An AppImage
+      is a self-running bundle that contains an application and everything
+      it needs to run that cannot reasonably expected to be part of each target system.
+    </p>
+    <p>
+      This can include libraries that are not commonly available on target systems,
+      resources such as translations, icons, fonts, and other auxiliary files.
+      appimagetool makes it easy to take an AppDir (e.g., generated by linuxdeployqt)
+      and turn it into an AppImage.
+    </p>
+    <p>
+      You can also embed update information so that your users can update the AppImage
+      using binary delta updates with the AppImageUpdate tool.
+      You can also sign the AppImage with GPG.
+    </p>
+  </description>
+  <url type="homepage">https://github.com/AppImage/AppImageKit</url>
+  <project_group>AppImage</project_group>
+  <provides>
+    <binary>appimagetool</binary>
+  </provides>
+</component>