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/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,