feat: add 404 page

app/not-found.tsx

@@ -0,0 +1,18 @@
+import Link from "next/link"
+
+export const metadata = { title: "Not Found" }
+
+export default function NotFound() {
+  return (
+    <main className="min-h-screen flex items-center justify-center px-6">
+      <div className="text-center">
+        <p className="font-mono text-sm text-ink-soft mb-4">404</p>
+        <h1 className="font-sans text-4xl font-bold tracking-tight text-ink mb-4">Page not found</h1>
+        <p className="text-ink-soft mb-8">The page you're looking for doesn't exist.</p>
+        <Link href="/" className="inline-block rounded-lg bg-ink px-6 py-3 text-sm font-medium text-canvas hover:opacity-80 transition-opacity">
+          Go home
+        </Link>
+      </div>
+    </main>
+  )
+}