feat(stats): resource metrics dashboard + sites logs/stats

Background collector samples CPU/memory/network/block I/O for every
instance and site on a configurable interval (default 15s, range
5-300s), persists samples to SQLite with a configurable retention
window (default 2h, range 0-24h), and skips ticks gracefully when
the Docker daemon is unreachable. Settings are reloadable without
a restart — each tick re-reads them.

New API endpoints:
- GET /api/system/stats (host snapshot: info + df)
- GET /api/system/stats/history
- GET /api/system/stats/top?by=cpu|memory
- GET /api/projects/{id}/stages/{s}/instances/{iid}/stats/history
- GET /api/sites/{id}/stats[/history]
- GET /api/sites/{id}/logs (SSE + JSON, reuses instance log streamer)

Frontend:
- ECharts added with tree-shaken imports (~180KB gzip) for
  future-proof time-series/gantt/graph visualizations
- CollapsibleSection wraps all dashboard sections (system health,
  daemons, system resources, static sites, projects) with
  localStorage-persisted open state
- SystemResourcesCard shows capacity tiles, workload utilization
  chart with 30m/2h/6h/24h window picker, disk breakdown with
  reclaimable callouts, and top 5 consumers
- ContainerStats and ContainerLogs take a source discriminated union
  so sites reuse the same components as instances; sites detail page
  embeds both for Deno backend debugging
- Settings › Maintenance exposes collection interval + retention
- Docker-unavailable state returns 503 and renders an amber banner
  instead of a generic 500

Full i18n coverage (en + ru) for all new strings.

web/src/lib/i18n/en.json

@@ -41,7 +41,47 @@
     "failedSites": "failed",
     "noSites": "No static sites yet.",
     "addFirstSite": "Deploy your first site",
-    "viewAllSites": "View all sites"
+    "viewAllSites": "View all sites",
+    "systemHealth": "System health",
+    "daemons": "Daemons",
+    "systemResources": "System resources",
+    "systemResourcesSubtitle": "CPU, memory, disk, and top consumers"
+  },
+  "resources": {
+    "cpuCores": "CPU Cores",
+    "memory": "Memory",
+    "running": "Running",
+    "dockerDisk": "Docker Disk",
+    "workloadUtilization": "Workload utilization",
+    "windowMinutes": "{n} minutes",
+    "windowHours": "{n} hours",
+    "noSamples": "No samples yet — the collector samples every {interval}s.",
+    "diskImages": "Images",
+    "diskContainers": "Containers",
+    "diskVolumes": "Volumes",
+    "diskBuildCache": "Build cache",
+    "reclaimable": "{size} reclaimable",
+    "topConsumers": "Top consumers",
+    "byCpu": "by cpu",
+    "byMemory": "by memory",
+    "noRunning": "No running containers.",
+    "instance": "instance",
+    "site": "site",
+    "showHistory": "Show history",
+    "hideHistory": "Hide history",
+    "cpuSeries": "CPU %",
+    "memorySeries": "Memory %",
+    "loading": "Loading…",
+    "sectionTitle": "Resources",
+    "showLogs": "Show logs",
+    "hideLogs": "Hide logs",
+    "dockerUnavailable": "Docker is unavailable. Check that the daemon is running."
+  },
+  "statsSettings": {
+    "intervalLabel": "Stats collection interval (s)",
+    "intervalHelp": "How often resource samples are collected. 0 disables collection. Range: 5–300s.",
+    "retentionLabel": "Stats retention (hours)",
+    "retentionHelp": "How long resource samples are kept. 0 disables collection. Range: 0–24h."
   },
   "projects": {
     "title": "Projects",