build(desktop): point electron-builder at new icon outputs

win.icon now references the hand-crafted multi-size .ico (16/24/32/48/
64/128/256) rather than letting electron-builder synthesise from a
single 1024 PNG — the latter was the cause of the 'Windows looks low
quality' complaint. linux.icon explicitly references build/icons/ dir
mode (was implicit before).

packages/desktop/electron-builder.yml

@@ -35,10 +35,11 @@ mac:
     - dmg
     - zip
 win:
-  icon: ./build/icon.png
+  icon: ./build/icon.ico
   target:
     - nsis
 linux:
+  icon: build/icons/
   target:
     - AppImage
     - deb