refactor: move shared utilities to lib/shared/ sub-packages

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

frontend/src/lib/notification/NotificationDropdown.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 import { m } from '$lib/paraglide/messages.js';
-import { relativeTime } from '$lib/utils/time';
+import { relativeTime } from '$lib/shared/utils/time';
 import type { NotificationItem } from '$lib/notification/notifications.svelte';
 
 type Props = {