Add React frontend, ZFS pool mapping, and multi-stage Docker build

- Vite + React frontend with dark-themed dashboard, slot grid per
  enclosure, and SMART detail overlay
- ZFS pool membership via zpool status -P
- Multi-stage Dockerfile (Node build + Python runtime)
- Updated docker-compose with network_mode host and healthcheck

frontend/package.json

@@ -0,0 +1,16 @@
+{
+  "name": "jbod-monitor-frontend",
+  "private": true,
+  "scripts": {
+    "dev": "vite",
+    "build": "vite build"
+  },
+  "dependencies": {
+    "react": "^18.3.1",
+    "react-dom": "^18.3.1"
+  },
+  "devDependencies": {
+    "@vitejs/plugin-react": "^4.3.4",
+    "vite": "^6.0.0"
+  }
+}