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/email/presentation/controller/single_email_controller.dart

@@ -952,6 +952,9 @@ class SingleEmailController extends BaseController with AppLoaderMixin {
         pressEmailAction(actionType, presentationEmail);
         break;
       case EmailActionType.labelAs:
+        if (!isLabelAvailable) {
+          return;
+        }
         mailboxDashBoardController.openAddLabelToEmailDialogModal(
           email: presentationEmail,
           onCreateANewLabelAction: () {