fix(#73): open annotation side panel when deep-linking via ?annotationId=

- NotificationBell now includes annotationId in the deep-link URL when available
- +page.svelte reads ?annotationId= param and sets activeAnnotationId on mount,
  opening the side panel instead of the bottom discussion drawer
- AnnotationSidePanel accepts and forwards targetCommentId to CommentThread
  so the specific comment is highlighted when navigating via a notification

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

frontend/src/lib/components/NotificationBell.svelte

@@ -9,6 +9,7 @@ type NotificationItem = {
 	type: 'REPLY' | 'MENTION';
 	documentId: string;
 	referenceId: string;
+	annotationId: string | null;
 	read: boolean;
 	createdAt: string;
 	actorName: string;
@@ -62,7 +63,9 @@ async function markRead(notification: NotificationItem) {
 			console.error('Failed to mark notification as read', e);
 		}
 	}
-	const url = `/documents/${notification.documentId}?commentId=${notification.referenceId}`;
+	const url = notification.annotationId
+		? `/documents/${notification.documentId}?commentId=${notification.referenceId}&annotationId=${notification.annotationId}`
+		: `/documents/${notification.documentId}?commentId=${notification.referenceId}`;
 	closeDropdown();
 	goto(url);
 }