fix: clamp restored pageIdx to valid range on doc change

app/page.js

@@ -95,6 +95,12 @@ export default function Home() {
             const doc = documents.find(d => d.index === selectedDocIndex);
             setCurrentDoc(doc);
             sessionStorage.setItem('selectedDocIndex', selectedDocIndex);
+
+            // Clamp pageIdx to valid range for this document
+            if (doc) {
+                const maxPage = (doc.annotatable_pages?.length ?? 1) - 1;
+                setPageIdx(prev => Math.min(prev, Math.max(0, maxPage)));
+            }
         }
     }, [selectedDocIndex, documents]);