Harden installer icon + NSIS rebuild path

- Regenerate icon.ico with classic BMP entries (16/32/48) — PNG-compressed
  ICO entries can fail NSIS' icon loader on some builds
- CI copies assets/textures/icon.ico onto the packaging path right before
  makensis, removing any stale/missing-copy failures
- Recreate v0.2.1 tag after this fix

.gitea/workflows/build.yml

@@ -83,6 +83,7 @@ jobs:
           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).
+          cp assets/textures/icon.ico packaging/windows/step-line.ico
           makensis \
             -DVERSION="$VERSION" \
             -DARTIFACT="build/step-line-windows-$VERSION.exe" \