revert(web): drop the 32x32 PNG favicon fallback

The SVG media query alone fixes the dark-mode icon for every SVG-capable
browser (Chrome, Edge, Firefox, Safari 26+); the PNG only served Safari
versions before 26, which is a shrinking legacy audience not worth a second
asset, its manifest/link declarations, and the extra test surface. index.html
and the manifest are back to the single SVG icon.

apps/web/index.html

@@ -4,7 +4,6 @@
     <meta charset="utf-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1" />
     <link rel="manifest" href="/manifest.webmanifest" />
-    <link rel="icon" type="image/png" sizes="32x32" href="/favicon-32x32.png" />
     <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
     <title>DeepSeek Harness</title>
   </head>