fix(app): add friendly not-found handling

apps/next/src/app/(app)/not-found.tsx

@@ -0,0 +1,17 @@
+import Link from 'next/link';
+
+import { Button } from '@spoon/ui';
+
+const NotFound = () => (
+  <main className='flex min-h-[50vh] flex-col items-center justify-center gap-4 p-6 text-center'>
+    <h1 className='text-2xl font-semibold'>Not found</h1>
+    <p className='text-muted-foreground max-w-md text-sm'>
+      This item does not exist, or you do not have access to it.
+    </p>
+    <Button asChild>
+      <Link href='/dashboard'>Back to dashboard</Link>
+    </Button>
+  </main>
+);
+
+export default NotFound;