fix: replace manifest icon paths with correctly sized 192 and 512 png assets

The manifest.json previously referenced a single icon file (snek1.png) for both 192x192 and 512x512 sizes, causing browser instability warnings due to mismatched dimensions. This commit adds properly sized snek192.png and snek512.png images and updates the manifest to point each icon entry to its corresponding size-specific asset, resolving the Firefox Android manifest validation issue.

src/snek/static/manifest.json

@@ -17,12 +17,12 @@
     "start_url": "/web.html",
     "icons": [
       {
-        "src": "/image/snek1.png",
+        "src": "/image/snek192.png",
         "type": "image/png",
         "sizes": "192x192"
       },
         {
-        "src": "/image/snek1.png",
+        "src": "/image/snek512.png",
         "type": "image/png",
         "sizes": "512x512"
       }