allow to move the selection panel via mouse drag

src/ui/selection/SelectionContent.h

@@ -45,6 +45,11 @@ public:
     // SelectionPanel owns that decision.
     void refresh();
 
+    // The card's header (REQ-UI-SELECTION-CARD), which is also the handle the panel is
+    // dragged by (REQ-UI-SELECTION-PANEL-DRAG). The panel hit-tests against it; the card
+    // itself does nothing with the gesture.
+    QWidget* getHeaderWidget() const { return m_header; }
+
 protected:
     // constructionSiteId is set only while the card shows a construction site, in which
     // case this base builds and drives the construction section in place of whatever the
@@ -91,6 +96,7 @@ private:
     // Set while this card shows a construction site rather than a finished object.
     std::optional<BuildingId> m_siteId;
 
+    QWidget*    m_header;
     QLabel*     m_symbolLabel;
     QLabel*     m_nameLabel;
     StatusPill* m_statusPill;