feat: show inventory timestamps on Device Inventory tiles

Surface the same lifecycle timestamps on the Device Inventory cards as on the
detail panel. Tiles for devices placed on a canvas show their linked node's
created / last scan / last modified / last seen (correlated by ip or
ieee_address, aggregated across matches: created = oldest, others = newest).
Devices not yet on any canvas fall back to their discovered_at.

Rendered as compact relative times ("2d ago") with the full date on hover, in
a tight two-column footer so the tile keeps its original footprint.

Backend: PendingDeviceResponse gains node_created_at / node_last_scan /
node_last_modified / node_last_seen; the canvas correlation now also pulls node
timestamps in the same single query. Frontend: shared timeFormat util
(absolute + relative), reused by the detail panel.

ha-relevant: maybe

frontend/src/components/panels/DetailPanel.tsx

@@ -8,19 +8,13 @@ import { NODE_TYPE_LABELS, STATUS_COLORS, type ServiceInfo, type ServiceStatus,
 import { getServiceUrl } from '@/utils/serviceUrl'
 import { splitIps } from '@/utils/maskIp'
 import { PROPERTY_ICONS, PROPERTY_ICON_NAMES, resolvePropertyIcon } from '@/utils/propertyIcons'
+import { formatTimestamp } from '@/utils/timeFormat'
 import type { Node } from '@xyflow/react'
 
 interface DetailPanelProps {
   onEdit: (id: string) => void
 }
 
-// Backend timestamps may arrive without a timezone suffix (naive UTC). Append
-// 'Z' when absent so the browser parses them as UTC rather than local time.
-function formatTimestamp(value: string): string {
-  const iso = /[Zz]|[+-]\d{2}:?\d{2}$/.test(value) ? value : value + 'Z'
-  return new Date(iso).toLocaleString()
-}
-
 type SvcForm = { port: string; protocol: 'tcp' | 'udp'; service_name: string; path: string }
 const EMPTY_FORM: SvcForm = { port: '', protocol: 'tcp', service_name: '', path: '' }