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/thread/presentation/thread_view.dart

@@ -651,11 +651,16 @@ class ThreadView extends GetWidget<ThreadController>
           dashboardController.toggleLabelToEmail(emailId, label, isSelected);
         },
         onCreateANewLabelAction: () {
+          final emailId = email.id;
+          if (emailId == null) {
+            logWarning('ThreadView::onCreateANewLabelAction: Email id is null');
+            return;
+          }
           dashboardController.labelController.handleLabelActionType(
             actionType: LabelActionType.create,
             accountId: dashboardController.accountId.value,
             onLabelActionCallback: (label) =>
-                dashboardController.toggleLabelToEmail(email.id!, label, true),
+                dashboardController.toggleLabelToEmail(emailId, label, true),
           );
         },
       ),