electron-builder's mac block declared 'icon: assets/icon.icns' but the
file was never generated, so macOS builds shipped with the default
Electron icon. The build would succeed but the .app looked generic.
Add scripts/build-icon-icns.js — a 50-line node script that packs the
existing PNGs from assets/icons/ (16/32/64/128/256/512/1024 + 48 small
+ 16/32/128/256 @2x retina) into a valid ICNS file. No native deps;
works on Linux, mac, and Windows runners alike (avoids the mac-only
'sips' that png2icns requires).
Wired into npm via 'npm run build:icon-icns'. Local build:linux now
parses the ICNS correctly and produces AppImage/deb/snap.
