feat(frontend): add React device status dashboard (#6)

frontend/.gitignore

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+node_modules/
+public/app.js
+public/app.css
+npm-debug.log*

frontend/README.md

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+# React device dashboard
+
+Minimal manager-facing frontend for the RPi-Monitoring `GET /devices` endpoint.
+
+## Develop and build
+
+From `frontend/` (Node 22 or newer):
+
+```sh
+npm install
+npm test
+npm run build
+python -m http.server 8080 --directory public
+```
+
+Open http://localhost:8080. API defaults to http://localhost:8000.
+Build-time override:
+
+```sh
+REACT_APP_API_BASE=https://monitor.example.com npm run build
+```
+
+On Windows PowerShell: `$env:REACT_APP_API_BASE="http://localhost:8000"; npm run build`.
+
+**Browser CORS:** A separate frontend origin such as localhost:8080 requires
+the FastAPI server to allow that origin or a same-origin reverse proxy.
+This frontend does not disable browser security or send credentials.
+
+The application is read-only. It handles initial loading, empty devices,
+HTTP errors, manual refresh, responsive layout and online/offline badges.
+`npm test` runs 7 isolated API tests with no hardware or backend.
+Generated `public/app.js` and `public/app.css` are gitignored.

frontend/build.mjs

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+import { build } from "esbuild";
+const apiBase = process.env.REACT_APP_API_BASE || "http://localhost:8000";
+await build({
+  entryPoints: ["src/index.jsx"],
+  outfile: "public/app.js",
+  bundle: true,
+  minify: true,
+  target: ["es2020"],
+  define: { "process.env.REACT_APP_API_BASE": JSON.stringify(apiBase) },
+  logLevel: "info",
+});
+console.log(`Built dashboard with API: ${apiBase}`);