refactor(notifications): extract NotificationItem type and relativeTime to shared utility

Extracted from NotificationBell.svelte into $lib/utils/notifications.ts so the
history page can reuse them. relativeTime() now accepts an optional `now` param
for deterministic unit testing. Added parseNotificationEvent() for SSE payload
shape validation (NullX Finding 3).

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

frontend/src/lib/components/NotificationBell.svelte

@@ -2,17 +2,11 @@
 import { onMount, onDestroy } from 'svelte';
 import { goto } from '$app/navigation';
 import { m } from '$lib/paraglide/messages.js';
-
-type NotificationItem = {
-	id: string;
-	type: 'REPLY' | 'MENTION';
-	documentId: string;
-	referenceId: string;
-	annotationId: string | null;
-	read: boolean;
-	createdAt: string;
-	actorName: string;
-};
+import {
+	type NotificationItem,
+	relativeTime,
+	parseNotificationEvent
+} from '$lib/utils/notifications';
 
 let notifications: NotificationItem[] = $state([]);
 let unreadCount: number = $state(0);
@@ -131,23 +125,12 @@ function attachClickOutside(node: HTMLElement) {
 	};
 }
 
-function relativeTime(isoString: string): string {
-	const diffMs = Date.now() - new Date(isoString).getTime();
-	const diffMin = Math.floor(diffMs / 60000);
-	const rtf = new Intl.RelativeTimeFormat(undefined, { numeric: 'auto' });
-	if (diffMin < 1) return rtf.format(0, 'minute');
-	if (diffMin < 60) return rtf.format(-diffMin, 'minute');
-	const diffH = Math.floor(diffMin / 60);
-	if (diffH < 24) return rtf.format(-diffH, 'hour');
-	const diffD = Math.floor(diffH / 24);
-	return rtf.format(-diffD, 'day');
-}
-
 onMount(() => {
 	fetchUnreadCount();
 	eventSource = new EventSource('/api/notifications/stream');
 	eventSource.addEventListener('notification', (e) => {
-		const notification = JSON.parse(e.data) as NotificationItem;
+		const notification = parseNotificationEvent(e.data);
+		if (!notification) return;
 		notifications = [notification, ...notifications];
 		if (!notification.read) unreadCount += 1;
 	});
@@ -317,6 +300,16 @@ onDestroy(() => {
 					{/each}
 				</ul>
 			{/if}
+
+			<div class="border-t border-line px-4 py-2">
+				<a
+					href="/notifications"
+					onclick={closeDropdown}
+					class="text-xs font-medium text-ink-2 transition-colors hover:text-ink"
+				>
+					{m.notification_view_all()}
+				</a>
+			</div>
 		</div>
 	{/if}
 </div>