feat: wire up master app icon for Electron builds

Point prebuild script at the 1024x1024 master icon.png at project root.
electron-builder auto-generates .icns (macOS), .ico (Windows), and
multi-size PNGs (Linux) from build/icon.png during packaging.

packages/desktop/electron-builder.yml

@@ -19,17 +19,14 @@ mac:
   target:
     - dmg
     - zip
-  # icon: build/icon.icns  # Uncomment when final icon is designed
 win:
   target:
     - nsis
-  # icon: build/icon.ico  # Uncomment when final icon is designed
 linux:
   target:
     - AppImage
     - deb
   category: Network
-  # icon: build/icons  # Uncomment when final icons are designed
 nsis:
   oneClick: false
   allowToChangeInstallationDirectory: true