viewer: add/subtract the selected ROI to/from the user mask

Restore the ROI-to-mask action on the new list: "Add to mask" and
"Subtract from mask" buttons rasterise the selected ROI into the user
mask (set or clear), through the same UpdateUserMask_i path. The ROI is
evaluated with per-pixel resolution and phi from the geometry, so box,
circle and azimuthal (sector) ROIs all map correctly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

viewer/JFJochViewerWindow.cpp

@@ -201,6 +201,8 @@ JFJochViewerWindow::JFJochViewerWindow(QWidget *parent, bool dbus, const QString
             reading_worker, &JFJochImageReadingWorker::DownloadROIsFromServer);
     connect(side_panel, &JFJochViewerSidePanel::uploadROIs,
             reading_worker, &JFJochImageReadingWorker::UploadROIsToServer);
+    connect(side_panel, &JFJochViewerSidePanel::maskFromROI,
+            reading_worker, &JFJochImageReadingWorker::MaskFromSelectedROI);
 
     connect(menuBar, &JFJochViewerMenu::clearUserMaskSelected,
             reading_worker, &JFJochImageReadingWorker::ClearUserMask);