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

@@ -452,18 +452,20 @@ function zoomOut() {
 			{/if}
 		</div>
 
-		{#if activeAnnotationId}
-			<AnnotationCommentPanel
-				documentId={documentId}
-				annotationId={activeAnnotationId}
-				canComment={canComment ?? false}
-				currentUserId={currentUserId ?? null}
-				canAdmin={canAdmin ?? false}
-				onClose={() => (activeAnnotationId = null)}
-				onCountChange={(count) => {
-					if (activeAnnotationId) commentCounts.set(activeAnnotationId, count);
-				}}
-			/>
-		{/if}
+		{#key activeAnnotationId}
+			{#if activeAnnotationId}
+				<AnnotationCommentPanel
+					documentId={documentId}
+					annotationId={activeAnnotationId}
+					canComment={canComment ?? false}
+					currentUserId={currentUserId ?? null}
+					canAdmin={canAdmin ?? false}
+					onClose={() => (activeAnnotationId = null)}
+					onCountChange={(count) => {
+						if (activeAnnotationId) commentCounts.set(activeAnnotationId, count);
+					}}
+				/>
+			{/if}
+		{/key}
 	</div>
 {/if}