Port to Qt6

- Update build configuration (CMake, QMake) to use Qt6 and C++17.
- Update Debian dependencies to use Qt6 packages.
- Replace `QRegExp` with `QRegularExpression` and `QRegExpValidator` with `QRegularExpressionValidator`.
- Replace deprecated `QSignalMapper::mapped` signal with `mappedString`.
- Replace `qsrand`/`qrand` with `QRandomGenerator`.
- Update `QProcess::start` to `QProcess::startCommand`.
- Update `QLibraryInfo::location` to `QLibraryInfo::path`.
- Update `QDataStream` version to `Qt_6_0`.
- Switch from `qt5keychain` to `qt6keychain`.
- Remove usage of `QDesktopWidget`.
- Replace `QString::midRef` with `QString::mid`.