fix: treat naive backend timestamps as UTC for relative labels

Backend emits store.Now() as "2026-04-23 14:05:32" — UTC but without a
Z marker or T separator. JavaScript parses such strings as local time,
so every "N ago" label is skewed by the browser's UTC offset (3h off
in UTC+3, causing the visible mismatch with the tooltip).

Normalise bare/space-separated timestamps to UTC inside toDate() so
every $fmt.* call, including relative, matches the absolute timestamp
shown in the tooltip. InstanceCard drops its own parser and delegates
to $fmt.relative for consistency.

web/src/lib/components/InstanceCard.svelte

@@ -9,6 +9,7 @@
 	import ConfirmDialog from './ConfirmDialog.svelte';
 	import { IconPlay, IconStop, IconRestart, IconTrash, IconExternalLink, IconEvents } from '$lib/components/icons';
 	import { t } from '$lib/i18n';
+	import { fmt } from '$lib/format/datetime';
 	import * as api from '$lib/api';
 
 	interface Props {
@@ -31,17 +32,7 @@
 			: instance.subdomain ? `https://${instance.subdomain}` : ''
 	);
 
-	const timeSinceCreated = $derived(() => {
-		const created = new Date(instance.created_at);
-		const now = new Date();
-		const diffMs = now.getTime() - created.getTime();
-		const diffMins = Math.floor(diffMs / 60_000);
-		if (diffMins < 60) return `${diffMins}m ago`;
-		const diffHours = Math.floor(diffMins / 60);
-		if (diffHours < 24) return `${diffHours}h ago`;
-		const diffDays = Math.floor(diffHours / 24);
-		return `${diffDays}d ago`;
-	});
+	const timeSinceCreated = $derived(() => $fmt.relative(instance.created_at));
 
 	async function handleAction(action: 'stop' | 'start' | 'restart' | 'remove') {
 		loading = true;