add dispose to files to remove controllers when not used

Frontend/patient_manager/lib/pages/fullScreenFile.dart

@@ -38,6 +38,12 @@ class _FullScreenFileViewerState extends State<FullScreenFileViewer> {
     });
   }
 
+  @override
+  void dispose() {
+    pdfViewerController.dispose();
+    super.dispose();
+  }
+
   @override
   void initState() {
     pdfViewerController.addListener(onPageSelect);