refactor: move notification domain to lib/notification/

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

frontend/src/lib/notification/notifications.ts

@@ -0,0 +1,31 @@
+export type NotificationItem = {
+	id: string;
+	type: 'REPLY' | 'MENTION';
+	documentId: string;
+	referenceId: string;
+	annotationId: string | null;
+	read: boolean;
+	createdAt: string;
+	actorName: string;
+	documentTitle: string | null;
+};
+
+export { relativeTime } from '$lib/utils/time';
+
+export function parseNotificationEvent(raw: string): NotificationItem | null {
+	try {
+		const parsed = JSON.parse(raw);
+		if (
+			typeof parsed.id !== 'string' ||
+			typeof parsed.documentId !== 'string' ||
+			typeof parsed.actorName !== 'string' ||
+			!['REPLY', 'MENTION'].includes(parsed.type)
+		) {
+			console.warn('Unexpected SSE payload shape:', parsed);
+			return null;
+		}
+		return parsed as NotificationItem;
+	} catch {
+		return null;
+	}
+}