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/AnnotationSidePanel.svelte

@@ -9,6 +9,7 @@ type Props = {
 	canComment: boolean;
 	currentUserId: string | null;
 	canAdmin: boolean;
+	targetCommentId?: string | null;
 	onClose: () => void;
 };
 
@@ -19,6 +20,7 @@ let {
 	canComment,
 	currentUserId,
 	canAdmin,
+	targetCommentId = null,
 	onClose
 }: Props = $props();
 
@@ -57,6 +59,7 @@ const visible = $derived(activeAnnotationId !== null);
 					canComment={canComment}
 					currentUserId={currentUserId}
 					canAdmin={canAdmin}
+					targetCommentId={targetCommentId}
 					loadOnMount={true}
 				/>
 			{/key}