feat(frontend): add polling job queue ui

frontend/README.md

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+# Frontend
+
+Vite React demo UI for the minimal job queue.
+
+It reuses the visual style from the previous advanced frontend, but only keeps:
+
+- dashboard
+- jobs page
+- job detail page
+- global events page
+
+The UI polls:
+
+- jobs and stats every 2 seconds
+- events every 1 second
+
+No WebSockets are used.
+
+## Run
+
+```powershell
+npm install
+npm run dev
+```
+
+The Docker frontend image serves the local `dist/` directory with nginx, so run `npm run build` before `docker compose up --build` if `dist/` is missing or stale.