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/main/routes/dialog_router.dart

@@ -41,8 +41,9 @@ class DialogRouter {
       );
 
       return returnedValue;
-    } catch (e) {
-      logWarning('DialogRouter::pushGeneralDialog: Exception = $e');
+    } catch (e, stackTrace) {
+      logWarning('DialogRouter::pushGeneralDialog: Exception = $e\n$stackTrace');
+      rethrow;
     } finally {
       if (PlatformInfo.isWeb) {
         _isMapDialogOpened.remove(routeName);
@@ -112,8 +113,9 @@ class DialogRouter {
         barrierLabel: dialogLabel,
         pageBuilder: (_, __, ___) => child,
       );
-    } catch (e) {
-      logWarning('DialogRouter::openDialogModal: Exception = $e');
+    } catch (e, stackTrace) {
+      logWarning('DialogRouter::openDialogModal: Exception = $e\n$stackTrace');
+      rethrow;
     } finally {
       if (PlatformInfo.isWeb && dialogLabel != null) {
         _isMapDialogOpened.remove(dialogLabel);