Fix NSIS installer build: absolute paths via defines, space-free installed exe, BMP ICO

- makensis resolves relative -DICON/-DARTIFACT against the script directory,
  so CI now passes $PWD-based absolute paths (reproduced locally in the same
  ubuntu:24.04 container; installer builds, 7z shows step-line.exe +
  uninstall.exe)
- Install as step-line.exe (no spaces: NSIS /oname splits on whitespace)
- OutFile overridable via -DOUT
- tools/generate_textures.py icon.ico now produced via ImageMagick with
  classic BMP entries (NSIS-compatible); PIL PNG-compressed ICOs rejected by
  NSIS

packaging/windows/step-line.nsi

@@ -14,8 +14,12 @@
   !define ICON "packaging\windows\step-line.ico"
 !endif
 
-; The installed executable always uses this clean name.
-!define INSTALLED_EXE "Step Line.exe"
+; The installed executable uses this clean name (no spaces — NSIS /oname tokens
+; on whitespace, so a spaced name breaks the File directive).
+!define INSTALLED_EXE "step-line.exe"
+!ifndef OUT
+  !define OUT "build\step-line-setup-${VERSION}.exe"
+!endif
 !define COMPANY "Step Line Project"
 !define UNINST_KEY "Software\Microsoft\Windows\CurrentVersion\Uninstall\Step Line"
 
@@ -25,7 +29,7 @@ Icon "${ICON}"
 UninstallIcon "${ICON}"
 
 Name "Step Line ${VERSION}"
-OutFile "build\step-line-setup-${VERSION}.exe"
+OutFile "${OUT}"
 InstallDir "$PROGRAMFILES64\Step Line"
 RequestExecutionLevel admin