MainWindow: Show hardcore mode error for recording playback

Show an error message when attempting to play an input recording while
RetroAchievements hardcore mode is active, instead of silently doing
nothing.

Source/Core/DolphinQt/MainWindow.cpp

@@ -1849,6 +1849,14 @@ void MainWindow::OnImportNANDBackup()
 
 void MainWindow::OnPlayRecording()
 {
+  if (AchievementManager::GetInstance().IsHardcoreModeActive())
+  {
+    ModalMessageBox::critical(
+        this, tr("Error"),
+        tr("Playback of input recordings is disabled in RetroAchievements hardcore mode."));
+    return;
+  }
+
   QString dtm_file = DolphinFileDialog::getOpenFileName(
       this, tr("Select the Recording File to Play"), QString(), tr("Dolphin TAS Movies (*.dtm)"));