refactor(time): extract relativeTime into shared time.ts utility

Move relativeTime from notifications.ts (Intl.RelativeTimeFormat) to a
new time.ts that uses the Paraglide comment_time_* message keys — the
same logic that was already in CommentThread's timeAgo(). Remove the
duplicate timeAgo() from CommentThread and re-export relativeTime from
notifications.ts for backwards compatibility.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/utils/time.ts

@@ -0,0 +1,12 @@
+import { m } from '$lib/paraglide/messages.js';
+
+export function relativeTime(isoString: string, now: Date = new Date()): string {
+	const diff = now.getTime() - new Date(isoString).getTime();
+	const minutes = Math.floor(diff / 60_000);
+	if (minutes < 1) return m.comment_time_just_now();
+	if (minutes < 60) return m.comment_time_minutes({ count: minutes });
+	const hours = Math.floor(minutes / 60);
+	if (hours < 24) return m.comment_time_hours({ count: hours });
+	const days = Math.floor(hours / 24);
+	return m.comment_time_days({ count: days });
+}