fix(desktop): ship multi-size icons so linux launchers can find them

electron-builder was given a single 2048x2048 icon.png and dumped it into
hicolor/2048x2048/apps/, a bucket no Linux desktop environment indexes —
launchers and taskbar fell back to a generic placeholder. Pre-render the
standard sizes (16, 32, 48, 64, 128, 256, 512, 1024) and point linux.icon
at the directory so each PNG lands in the matching hicolor/NxN/apps/ slot.

Tray icon was unaffected (loaded at runtime via Electron's Tray API from
bundled resources, no theme lookup).

surfsense_desktop/electron-builder.yml

@@ -77,7 +77,7 @@ nsis:
   createDesktopShortcut: true
   createStartMenuShortcut: true
 linux:
-  icon: assets/icon.png
+  icon: assets/icons/
   category: Utility
   artifactName: "${productName}-${version}-${arch}.${ext}"
   mimeTypes: