feat(extension): add shared relative-time util with tests

extension/src/shared/relative-time.ts

@@ -0,0 +1,27 @@
+/// Single source of truth for relative-time formatting and trash-retention math.
+/// Pulled out of five near-duplicate inline copies (settings-vault, devices,
+/// trash, field-history, vault/vault).
+
+import type { TrashRetention } from './types';
+
+/// Format a past unix timestamp (seconds) as "Nm ago" / "Nh ago" / "Nd ago" /
+/// "Nw ago" / "Nmo ago" relative to now. Returns "just now" under 60 seconds.
+export function relativeTime(unixSec: number): string {
+  const now = Math.floor(Date.now() / 1000);
+  const diff = now - unixSec;
+  if (diff < 60)      return 'just now';
+  if (diff < 3600)    return `${Math.floor(diff / 60)}m ago`;
+  if (diff < 86400)   return `${Math.floor(diff / 3600)}h ago`;
+  if (diff < 604800)  return `${Math.floor(diff / 86400)}d ago`;
+  if (diff < 2592000) return `${Math.floor(diff / 604800)}w ago`;
+  return `${Math.floor(diff / 2592000)}mo ago`;
+}
+
+/// Days remaining until an item trashed at `trashedAt` (unix seconds) will be
+/// auto-purged given the vault's retention policy. Returns null for forever
+/// retention; clamps to 0 when the retention window has already elapsed.
+export function daysUntilPurge(trashedAt: number, retention: TrashRetention): number | null {
+  if (retention.kind === 'forever') return null;
+  const trashedDaysAgo = Math.floor((Date.now() / 1000 - trashedAt) / 86400);
+  return Math.max(0, retention.value - trashedDaysAgo);
+}