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

.gitea/workflows/build.yml

@@ -81,9 +81,12 @@ jobs:
         run: |
           sudo apt-get update && sudo apt-get install -y nsis p7zip-full
           VERSION="$(sed -n 's/.*config\/version="\([^"]*\)".*/\1/p' project.godot)"
-          makensis /DVERSION="$VERSION" \
-            /DARTIFACT="build/step-line-windows-$VERSION.exe" \
-            /DICON=packaging/windows/step-line.ico \
+          # NOTE: on Linux, makensis uses -D defines (the /D form is Windows-only
+          # and is rejected as a script path).
+          makensis \
+            -DVERSION="$VERSION" \
+            -DARTIFACT="build/step-line-windows-$VERSION.exe" \
+            -DICON=packaging/windows/step-line.ico \
             packaging/windows/step-line.nsi
 
       - name: Verify Windows installer contents