fix(frontend): import React for JSX runtime

Prevent the Vite dev app from rendering a blank page by ensuring React is available in App.jsx when JSX is transformed.

frontend/src/App.jsx

@@ -9,7 +9,7 @@ import {
   Trash2,
   Upload,
 } from "lucide-react";
-import { useEffect, useMemo, useState } from "react";
+import React, { useEffect, useMemo, useState } from "react";
 
 const API_BASE = import.meta.env.VITE_API_BASE_URL || "http://localhost:8000";