Add System Trends sparkline charts using Recharts + Prometheus

- Add recharts dependency for area chart visualizations
- Add /api/metrics route querying Prometheus query_range (CPU, RAM, Network)
- Add SystemTrends component with 3 sparkline area charts (6h window, 2min steps)
- CPU (emerald), RAM (amber), Network I/O (indigo) with gradient fills
- Tooltips show exact values on hover, time axis with formatted labels
- Link to Grafana for deep-dive analysis
- Fetches every 60s, shimmer loading state

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -13,7 +13,8 @@
     "@tanstack/react-table": "^8.21.3",
     "next": "16.1.6",
     "react": "19.2.3",
-    "react-dom": "19.2.3"
+    "react-dom": "19.2.3",
+    "recharts": "^3.7.0"
   },
   "devDependencies": {
     "@tailwindcss/postcss": "^4",