add proper square Windows icon with multiple sizes

The source PNG (1536x1024) was non-square, causing the icon to appear
squished on Windows. Center-crop to 1024x1024 and generate a multi-size
.ico (16, 32, 48, 64, 128, 256) for correct display across taskbar,
title bar, alt-tab, and explorer.

package.json

@@ -158,7 +158,7 @@
         "nsis",
         "zip"
       ],
-      "icon": "assets/SubMiner.png"
+      "icon": "assets/SubMiner.ico"
     },
     "nsis": {
       "oneClick": false,