fix: 修复 Qt 5.12 中 Qt::endl 兼容性问题

Qt::endl 不是 Qt 命名空间成员，改用 '\n'

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/utils/logger.cpp

@@ -119,7 +119,7 @@ void Logger::writeToFile(const QString& line) {
     if (logFile_ && logFile_->isOpen()) {
         QTextStream stream(logFile_);
         stream.setCodec("UTF-8");
-        stream << line << Qt::endl;
+        stream << line << '\n';
         logFile_->flush();
     }
 }