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

.gitea/workflows/build.yml

@@ -82,19 +82,21 @@ jobs:
           sudo apt-get update && sudo apt-get install -y nsis p7zip-full
           VERSION="$(sed -n 's/.*config\/version="\([^"]*\)".*/\1/p' project.godot)"
           # NOTE: on Linux, makensis uses -D defines (the /D form is Windows-only
-          # and is rejected as a script path).
+          # and is rejected as a script path), and it resolves relative paths
+          # against the script directory — pass absolute paths from the workspace.
           cp assets/textures/icon.ico packaging/windows/step-line.ico
           makensis \
             -DVERSION="$VERSION" \
-            -DARTIFACT="build/step-line-windows-$VERSION.exe" \
-            -DICON=packaging/windows/step-line.ico \
+            -DARTIFACT="$PWD/build/step-line-windows-$VERSION.exe" \
+            -DICON="$PWD/packaging/windows/step-line.ico" \
+            -DOUT="$PWD/build/step-line-setup-$VERSION.exe" \
             packaging/windows/step-line.nsi
 
       - name: Verify Windows installer contents
         run: |
           VERSION="$(sed -n 's/.*config\/version="\([^"]*\)".*/\1/p' project.godot)"
           ls -la build/step-line-setup-$VERSION.exe
-          7z l build/step-line-setup-$VERSION.exe | grep -E "Step Line.exe|uninstall.exe"
+          7z l build/step-line-setup-$VERSION.exe | grep -E "step-line.exe|uninstall.exe"
 
       - name: Package Linux AppImage
         run: |