New notebook logo: spiral-bound page icon for favicon/PWA/desktop

SVG source in frontend/public/icon.svg, rendered to 192/512/maskable PNGs
and desktop/build/appicon.png; SVG favicon with PNG fallback; icons added
to the service-worker precache.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/index.html

@@ -6,7 +6,8 @@
     <meta name="theme-color" content="#1a1a1a" media="(prefers-color-scheme: dark)" />
     <meta name="theme-color" content="#fafafa" media="(prefers-color-scheme: light)" />
     <link rel="manifest" href="/manifest.webmanifest" />
-    <link rel="icon" href="/icon-192.png" />
+    <link rel="icon" type="image/svg+xml" href="/icon.svg" />
+    <link rel="icon" type="image/png" href="/icon-192.png" />
     <link rel="apple-touch-icon" href="/icon-192.png" />
     <title>Tefter</title>
   </head>