ifcviewer-full: federation settings dialog (unit + false origin)

New FederationSettingsDialog edits the federation-wide unit and the
FederatedFalseOrigin (XYZ + Z-rotation in that unit).  On Ok it calls
Federation::setConfig + setFederatedFalseOrigin, which fire the
granular Federation signals MainWindow listens to → viewport
recomposes immediately.

Reachable from File > Federation Settings.  Unit picker is a fixed
combobox of common length units (metres / mm / cm / km / ft / in /
yd / mi); each item carries (prefix, name) in itemData so saving
round-trips correctly.  Per-model ModelTransformation editor still
to come — that's a per-model dialog reachable from the model entry,
not the federation-wide settings.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer-full/MainWindow.h

@@ -36,6 +36,7 @@
 
 class Federation;
 class SettingsWindow;
+class FederationSettingsDialog;
 
 class MainWindow : public QMainWindow {
     Q_OBJECT
@@ -61,6 +62,7 @@ private slots:
     void onSetHomeView();
     void onGoHomeView();
     void onFileSettings();
+    void onFederationSettings();
     void onObjectPicked(uint32_t object_id);
     void onTreeSelectionChanged();
 
@@ -119,6 +121,7 @@ private:
     SceneLoader*    loader_   = nullptr;
     Federation*     federation_ = nullptr;
     SettingsWindow* settings_ = nullptr;
+    FederationSettingsDialog* federation_settings_ = nullptr;
     QWidget* viewport_container_ = nullptr;
     QTreeWidget* element_tree_ = nullptr;
     QTableWidget* property_table_ = nullptr;