Add React frontend and Sinbad2IA LLM integration.

Introduce a full Vite/React UI for exams, auth, materials, images, generation, and export.
Adapt backend for Sinbad2IA chat API, bcrypt passwords, CORS on port 5173, and schema migrations.

frontend/src/pages/NotFoundPage.jsx

@@ -0,0 +1,19 @@
+import { Link } from "react-router-dom";
+import Icon from "../components/ui/Icon";
+
+export default function NotFoundPage() {
+  return (
+    <div className="page page-narrow text-center" style={{ paddingTop: 90 }}>
+      <div className="icon-wrap icon-box icon-box-lg" style={{ margin: "0 auto 16px" }}>
+        <Icon name="compass" size={32} className="icon-muted" />
+      </div>
+      <h1 style={{ fontSize: 30 }}>Página no encontrada</h1>
+      <p className="text-soft">
+        La página que buscas no existe o ha sido movida.
+      </p>
+      <Link to="/" className="btn btn-primary mt">
+        Volver al inicio
+      </Link>
+    </div>
+  );
+}