feat(frontend): add toast notifications for dashboard actions

- Install react-hot-toast library
- Add Toaster component to App.jsx
- Show success/error toasts for match accept/reject/cancel
- Show toasts for real-time match events
- Update tests with toast mocks

frontend/package.json

@@ -18,6 +18,7 @@
     "qrcode.react": "^4.2.0",
     "react": "^19.2.0",
     "react-dom": "^19.2.0",
+    "react-hot-toast": "^2.6.0",
     "react-router-dom": "^7.9.5",
     "socket.io-client": "^4.8.1"
   },