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/src/lib/api/stats.ts

@@ -0,0 +1,44 @@
+import { apiFetch, type ApiResult } from '$lib/api/client';
+
+export type TimeRange = '5m' | '1h' | '6h' | '24h' | '30d';
+
+export type StatsResponse = {
+	range: TimeRange;
+	current: {
+		running_count: number;
+		vcpus_reserved: number;
+		memory_mb_reserved: number;
+		sampled_at?: string;
+	};
+	peaks: {
+		running_count: number;
+		vcpus: number;
+		memory_mb: number;
+	};
+	series: {
+		labels: string[];
+		running: number[];
+		vcpus: number[];
+		memory_mb: number[];
+	};
+};
+
+export async function fetchStats(range: TimeRange): Promise<ApiResult<StatsResponse>> {
+	return apiFetch('GET', `/api/v1/sandboxes/stats?range=${range}`);
+}
+
+export const POLL_INTERVALS: Record<TimeRange, number> = {
+	'5m':  15_000,
+	'1h':  30_000,
+	'6h':  60_000,
+	'24h': 120_000,
+	'30d': 300_000,
+};
+
+export const RANGE_LABELS: Record<TimeRange, string> = {
+	'5m':  '5m',
+	'1h':  '1h',
+	'6h':  '6h',
+	'24h': '24h',
+	'30d': '30d',
+};