refactored HTTP error handling and streamlined UI components by centralizing shared logic and removing redundant parameters

lib/app.dart

@@ -256,9 +256,7 @@ class _AppState extends State<App> with WidgetsBindingObserver {
 
       if (totalTabs != _knownTotalTabs) {
         var targetIndex = currentIndex;
-        if (_userOnLastTab) {
-          targetIndex = totalTabs - 1;
-        } else if (currentIndex >= totalTabs) {
+        if (_userOnLastTab || currentIndex >= totalTabs) {
           targetIndex = totalTabs - 1;
         }
         // Replace the controller atomically: a stale index past the new