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

packaging/linux/build_appimage.sh

@@ -27,8 +27,11 @@ cp "$HERE/step-line.desktop" "$APPDIR/step-line.desktop"
 cp "$HERE/AppRun" "$APPDIR/AppRun"
 chmod +x "$APPDIR/AppRun"
 cp "$HERE/step-line.png" "$APPDIR/step-line.png"
-mkdir -p "$APPDIR/usr/share/metainfo"
-cp "$HERE/step-line.appdata.xml" "$APPDIR/usr/share/metainfo/step-line.appdata.xml"
+# Desktop entry + metainfo in standard locations (AppStream validation requires
+# the metainfo filename to match the component id, and the .desktop discoverable).
+mkdir -p "$APPDIR/usr/share/applications" "$APPDIR/usr/share/metainfo"
+cp "$HERE/step-line.desktop" "$APPDIR/usr/share/applications/step-line.desktop"
+cp "$HERE/step-line.appdata.xml" "$APPDIR/usr/share/metainfo/net.stepline.game.appdata.xml"
 
 TOOL="${APPIMAGE_TOOL:-/tmp/appimagetool-x86_64.AppImage}"
 if [[ ! -f "$TOOL" ]]; then

packaging/linux/step-line.appdata.xml

@@ -1,8 +1,10 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <component type="desktop-application">
-  <id>net.step-line.StepLine</id>
+  <id>net.stepline.game</id>
   <name>Step Line</name>
   <summary>Workplace security simulation on Hollywood Boulevard</summary>
+  <url type="homepage">https://git.allucanget.biz/zwitschi/security-game</url>
+  <developer id="net.stepline"><name>Step Line Team</name></developer>
   <description>
     <p>
       Step Line is a workplace security simulation about observation, communication,