fix: dark background + emoji favicon for rStack design consistency

Standardize to dark background (#0f172a / slate-900) and add emoji
data URI favicon matching the rStack ecosystem.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/layout.tsx

@@ -20,7 +20,7 @@ export const metadata: Metadata = {
   },
   manifest: '/manifest.json',
   icons: {
-    icon: '/favicon.ico',
+    icon: "data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'><text y='.9em' font-size='90'>🗺️</text></svg>",
     apple: '/apple-touch-icon.png',
   },
 };