Fix Qt 6.9 compatibility: Replace qt_error_string() with enum mapping

The internal Qt function qt_error_string() was removed in Qt 6.9.
This commit replaces it with an idiomatic switch statement that maps
QFileDevice::FileError enum values to human-readable strings.

- Uses tr() for localization support
- Provides clear error messages (e.g., "Permissions error" vs error codes)
- Standard C++ approach when Qt enums aren't Q_ENUM-registered
- All FileError cases covered with default fallback

Tested with Qt 6.9.3 on macOS arm64.

Author: Entrpi

src/NotepadNext/dialogs/MainWindow.cpp

@@ -1888,7 +1888,29 @@ bool MainWindow::checkFileForModification(ScintillaNext *editor)
 void MainWindow::showSaveErrorMessage(ScintillaNext *editor, QFileDevice::FileError error)
 {
     const QString name = editor->isFile() ? editor->getFilePath() : editor->getName();
-    QMessageBox::warning(this, tr("Error Saving File"), tr("An error occurred when saving <b>%1</b><br><br>Error: %2").arg(name, qt_error_string(error)));
+
+    // Map error code to human-readable string
+    QString errorString;
+    switch (error) {
+        case QFileDevice::ReadError:        errorString = tr("Read error"); break;
+        case QFileDevice::WriteError:       errorString = tr("Write error"); break;
+        case QFileDevice::FatalError:       errorString = tr("Fatal error"); break;
+        case QFileDevice::ResourceError:    errorString = tr("Resource error"); break;
+        case QFileDevice::OpenError:        errorString = tr("Open error"); break;
+        case QFileDevice::AbortError:       errorString = tr("Abort error"); break;
+        case QFileDevice::TimeOutError:     errorString = tr("Timeout error"); break;
+        case QFileDevice::UnspecifiedError: errorString = tr("Unspecified error"); break;
+        case QFileDevice::RemoveError:      errorString = tr("Remove error"); break;
+        case QFileDevice::RenameError:      errorString = tr("Rename error"); break;
+        case QFileDevice::PositionError:    errorString = tr("Position error"); break;
+        case QFileDevice::ResizeError:      errorString = tr("Resize error"); break;
+        case QFileDevice::PermissionsError: errorString = tr("Permissions error"); break;
+        case QFileDevice::CopyError:        errorString = tr("Copy error"); break;
+        default:                            errorString = tr("Unknown error (%1)").arg(static_cast<int>(error)); break;
+    }
+
+    QMessageBox::warning(this, tr("Error Saving File"),
+        tr("An error occurred when saving <b>%1</b><br><br>Error: %2").arg(name, errorString));
 }
 
 void MainWindow::showEditorZoomLevelIndicator()