Ship a full icon set so KDE resolves the launcher icon

The Linux packages only installed a single 1024x1024 hicolor PNG, and
Plasma's icon lookup returns nothing for a lone oversized icon — confirmed
with kiconfinder6 under Papirus-Dark. Generate 16-1024 px PNGs from the
favicon and point electron-builder at the directory so every standard
hicolor size is installed.

desktop/electron-builder.yml

@@ -35,7 +35,7 @@ linux:
     J621 is a self-hosted browser for an e621-linked media library. This
     desktop build talks to a J621 backend over the network, the same way the
     web app does.
-  icon: build/icon.png
+  icon: build/icons
   syncDesktopName: true
 
 pacman: