feat(frontend): upgrade React 17→18 + react-scripts 4→5 (Phase 2)

- Update package.json: react/react-dom ^17→^18.3.1, react-scripts 4→5.0.1
- Migrate to React 18 Root API in src/index.js (createRoot)
- Add --legacy-peer-deps to Dockerfile for MUI v4 compatibility
- Regenerate package-lock.json with legacy peer deps flag

✅ Tested: Production build 253.28 KB gzip, containers running
✅ Manual test: Upload, Moderation, Public View, Slideshow - all working

Phase 2 complete: Frontend on React 18 with concurrent rendering.

frontend/Dockerfile

@@ -2,7 +2,7 @@
 FROM node:18-alpine AS build
 WORKDIR /app
 COPY package.json ./
-RUN npm install --silent
+RUN npm install --silent --legacy-peer-deps
 COPY . ./
 ENV NODE_OPTIONS=--openssl-legacy-provider
 RUN npm run build

frontend/package.json

@@ -9,14 +9,14 @@
     "@testing-library/react": "^11.2.5",
     "@testing-library/user-event": "^12.8.3",
     "axios": "^0.21.1",
-    "react": "^17.0.1",
+    "react": "^18.3.1",
+    "react-dom": "^18.3.1",
     "react-code-blocks": "^0.0.8",
-    "react-dom": "^17.0.1",
     "react-dropzone": "^11.3.1",
     "react-helmet": "^6.1.0",
     "react-lottie": "^1.2.3",
     "react-router-dom": "^5.2.0",
-    "react-scripts": "4.0.3",
+    "react-scripts": "5.0.1",
     "sass": "^1.32.8",
     "sweetalert2": "^10.15.6",
     "web-vitals": "^1.1.0"

frontend/src/index.js

@@ -1,11 +1,11 @@
 import React from 'react';
-import ReactDOM from 'react-dom';
+import ReactDOM from 'react-dom/client';
 import App from './App';
 import './App.css';
 
-ReactDOM.render(
+const root = ReactDOM.createRoot(document.getElementById('root'));
+root.render(
   <React.StrictMode>
     <App />
-  </React.StrictMode>,
-  document.getElementById('root')
+  </React.StrictMode>
 );