fix(comments): remount AnnotationCommentPanel when switching annotations

Wrap the panel in {#key activeAnnotationId} so Svelte destroys and
recreates it on every annotation change, triggering onMount and
loading the correct comments.

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

frontend/src/lib/components/PdfViewer.svelte

@@ -452,6 +452,7 @@ function zoomOut() {
 			{/if}
 		</div>
 
+		{#key activeAnnotationId}
 			{#if activeAnnotationId}
 				<AnnotationCommentPanel
 					documentId={documentId}
@@ -465,5 +466,6 @@ function zoomOut() {
 					}}
 				/>
 			{/if}
+		{/key}
 	</div>
 {/if}