Add React race hub frontend

frontend/README.md

@@ -0,0 +1,33 @@
+# box-box Web Frontend
+
+React Race Hub slice for the production Web UI.
+
+## Commands
+
+```bash
+npm install
+npm run dev
+npm test -- --run
+npm run build
+```
+
+The Vite dev server proxies `/api` requests to the Go server on
+`http://localhost:8080`.
+
+Run the backend separately:
+
+```bash
+go run cmd/main.go --web
+```
+
+Then open the React app, usually:
+
+```text
+http://localhost:5173/race-hub
+```
+
+Load an ingested session with:
+
+```text
+http://localhost:5173/race-hub?session_key=9472
+```