fix(web): make the favicon follow the color scheme

favicon.svg embeds @media (prefers-color-scheme: dark) so the DeepSeek
mark turns white under a dark browser scheme; index.html and the install
manifest gain a 32x32 PNG fallback (brand blue) for Safari, which cannot
render SVG favicons. pwa-manifest.e2e.ts pins the PNG link, both manifest
icons, and the dark media query.

apps/web/index.html

@@ -4,6 +4,7 @@
     <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>