allow custom oneko image via themes - fixes #472

after this change, one can set a custom image by:

* upload an appropriate image to the drive (some images can be
  found at
	https://github.com/vencordcss/onekocord/tree/main/onekoskins),
	possibly with the "keep original" option set
* copy the URL to the image in the drive
* create/edit a theme so that it contains (inside `props`):

     "oneko-image": '"url(https://yourinstance.example.com/files/ee17b385-a084-4e2a-b531-225dfb96cc3c)',

  with the proper URL

That's it!

packages/frontend/src/components/SkOneko.vue

@@ -235,6 +235,6 @@ onMounted(init);
 	pointer-events: none;
 	image-rendering: pixelated;
 	z-index: 2147483647;
-	background-image: url(/client-assets/oneko.gif);
+	background-image: var(--oneko-image, url(/client-assets/oneko.gif));
 }
 </style>