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/mixin/label_modal_mixin.dart

@@ -12,8 +12,8 @@ import 'package:tmail_ui_user/main/routes/dialog_router.dart';
 mixin LabelModalMixin {
   List<Label> get labels;
   ImagePaths get imagePaths;
-  CreateNewLabelInteractor? get createNewLabelInteractor;
-  EditLabelInteractor? get editLabelInteractor;
+  CreateNewLabelInteractor get createNewLabelInteractor;
+  EditLabelInteractor get editLabelInteractor;
 
   Future<dynamic> openCreateNewLabelModal({
     required AccountId accountId,
@@ -26,8 +26,8 @@ mixin LabelModalMixin {
         accountId: accountId,
         imagePaths: imagePaths,
         verifyNameInteractor: verifyNameInteractor,
-        createNewLabelInteractor: createNewLabelInteractor!,
-        editLabelInteractor: editLabelInteractor!,
+        createNewLabelInteractor: createNewLabelInteractor,
+        editLabelInteractor: editLabelInteractor,
       ),
       dialogLabel: 'create-new-label-modal',
     );
@@ -47,8 +47,8 @@ mixin LabelModalMixin {
         selectedLabel: selectedLabel,
         actionType: LabelActionType.edit,
         verifyNameInteractor: verifyNameInteractor,
-        createNewLabelInteractor: createNewLabelInteractor!,
-        editLabelInteractor: editLabelInteractor!,
+        createNewLabelInteractor: createNewLabelInteractor,
+        editLabelInteractor: editLabelInteractor,
       ),
       dialogLabel: 'edit-label-modal',
     );