fix: use explicit children type in RootLayout (CI typecheck)

LayoutProps<'/'> is a Next-generated global type that only exists after
next typegen/build creates .next/types. Fresh CI clones fail tsc --noEmit
because .next doesn't exist yet. Use Readonly<{children}> — no dependency
on generated types.

app/layout.tsx

@@ -18,7 +18,7 @@ export const metadata: Metadata = {
   description: "Generated by create next app",
 };
 
-export default function RootLayout({ children }: LayoutProps<"/">) {
+export default function RootLayout({ children }: Readonly<{ children: React.ReactNode }>) {
   return (
     <html
       lang="en"