Fix blank /students/new and /students/$id (missing Outlet)

students.tsx was the list AND the parent of students.new / students.$id but
rendered no <Outlet/>, so child routes never showed. Make students.tsx an
Outlet layout and move the list to students.index.tsx. Add an errorComponent
to students.new as a safety net.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/routes/_authenticated/students.new.tsx

@@ -15,6 +15,12 @@ import { toast } from "sonner";
 export const Route = createFileRoute("/_authenticated/students/new")({
   head: () => ({ meta: [{ title: "New student — School Portal" }] }),
   component: NewStudentPage,
+  errorComponent: ({ error }) => (
+    <div className="p-8 max-w-2xl">
+      <h1 className="text-lg font-semibold text-destructive">This page hit an error</h1>
+      <pre className="mt-3 whitespace-pre-wrap break-words text-xs bg-muted p-3 rounded border">{error?.message}{"\n\n"}{error?.stack}</pre>
+    </div>
+  ),
 });
 
 type Guardian = {