Add live stats page with metrics sampling and route split

- New sandbox_metrics_snapshots table sampled every 10s (60-day retention)
- Background MetricsSampler goroutine wired into control plane startup
- GET /v1/sandboxes/stats?range=5m|1h|6h|24h|30d endpoint with adaptive
  polling intervals; reserved CPU/RAM uses ceil(paused/2) formula
- StatsPanel component: 4 stat cards + 2 Chart.js line charts (straight
  lines, integer y-axis for running count, dual-axis for CPU/RAM)
- Range filter persisted in URL query param; polls update data silently
  (no blink — loading state only shown on initial mount)
- Split /dashboard/capsules into /list and /stats sub-routes with shared
  layout; capsuleRunningCount store syncs badge across routes
- CreateCapsuleDialog extracted as reusable component

frontend/package.json

@@ -26,5 +26,8 @@
 		"tailwindcss": "^4.2.1",
 		"typescript": "^5.9.3",
 		"vite": "^7.3.1"
+	},
+	"dependencies": {
+		"chart.js": "^4.5.1"
 	}
 }