fix: anchor unmatched URLs into main so 404 renders

app/(main)/[...rest]/page.tsx

@@ -0,0 +1,10 @@
+import { notFound } from 'next/navigation';
+
+// Catch-all that anchors unmatched URLs into the (main) route group so
+// Next renders app/(main)/not-found.tsx (wrapped by (main)/layout.tsx)
+// instead of the built-in __next_builtin__not-found page. Without this,
+// route groups can't pick a root layout for URLs that match nothing, so
+// 404s render bare.
+export default function CatchAll() {
+  notFound();
+}