Use the header's Terminal icon as the app favicon

Replaces the default create-next-app favicon.ico with an icon.svg
matching lucide-react's Terminal glyph (same one used in the header)
exactly, on a dark rounded-square backdrop so it stays legible at
16x16 in the browser tab regardless of the OS/browser chrome theme.

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

app/src/app/icon.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" width="24" height="24">
+  <rect width="24" height="24" rx="5" fill="#09090b" />
+  <path d="M12 19h8" stroke="#fafafa" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" fill="none" />
+  <path d="m4 17 6-6-6-6" stroke="#fafafa" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" fill="none" />
+</svg>