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/components/layout/Layout.jsx

@@ -0,0 +1,11 @@
+import { Outlet } from "react-router-dom";
+import Navbar from "./Navbar";
+
+export default function Layout() {
+  return (
+    <div className="app-shell">
+      <Navbar />
+      <Outlet />
+    </div>
+  );
+}