TF-4301 Address code review comments on label feature

- Make LabelModalMixin interactor getters non-nullable; remove force-unwraps at call sites
- Show positiveLabel and add semanticLabel in progressing button for accessibility
- Use timestamp-based label name in E2E scenario to prevent flakiness on reused accounts
- Guard labelAs action with isLabelAvailable check in SingleEmailController
- Don't pop back on label creation failure so users can retry without losing context
- Block modal dismissal while label creation request is in flight
- Null-guard email.id before toggling label in ThreadView::onCreateANewLabelAction
- Log stackTrace and rethrow in DialogRouter to distinguish errors from user cancel

lib/features/labels/presentation/label_controller.dart

@@ -107,10 +107,10 @@ class LabelController extends BaseController
   }
 
   @override
-  CreateNewLabelInteractor? get createNewLabelInteractor => _createNewLabelInteractor;
+  CreateNewLabelInteractor get createNewLabelInteractor => _createNewLabelInteractor!;
 
   @override
-  EditLabelInteractor? get editLabelInteractor => _editLabelInteractor;
+  EditLabelInteractor get editLabelInteractor => _editLabelInteractor!;
 
   DeleteALabelInteractor? get deleteALabelInteractor => _deleteALabelInteractor;
 
@@ -221,9 +221,6 @@ class LabelController extends BaseController
     bool shouldPop = false,
   }) {
     toastManager.showMessageFailure(failure);
-    if (shouldPop) {
-      popBack();
-    }
   }
 
   void _addLabelToList(Label newLabel) {