refactor(notifications): import relativeTime from canonical time.ts

NotificationDropdown was importing relativeTime through notifications.ts,
creating an accidental coupling to a module unrelated to timestamp formatting.
Now imports directly from the canonical \$lib/utils/time module.

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

frontend/src/lib/components/NotificationDropdown.svelte

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