fix(web): add favicon.ico and metadata icon reference

Add a mosaic-pattern favicon.ico (16x16, 32x32, 48x48 multi-size ICO)
to apps/web/src/app/ using Next.js App Router convention. Update
metadata export in layout.tsx to include explicit icon reference.
Resolves 404 for /favicon.ico on mosaic.woltje.com.

Closes #534

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

apps/web/src/app/layout.tsx

@@ -11,6 +11,9 @@ export const dynamic = "force-dynamic";
 export const metadata: Metadata = {
   title: "Mosaic Stack",
   description: "Mosaic Stack Web Application",
+  icons: {
+    icon: "/favicon.ico",
+  },
 };
 
 const outfit = Outfit({