Add recent projects to Bonsai Viewer

Replace the "Open Recent coming soon" placeholder with a working
most-recently-used project list. RecentProjects persists .ifcfed paths
via QSettings, capped and pruned to existing files. The Open Recent
ribbon button now shows a popup menu of recent projects; every
successful open or save (local or cloud) records an entry.

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

src/bonsaiviewer/MainWindow.h

@@ -27,6 +27,7 @@
 
 class QLabel;
 class QDockWidget;
+class QMenu;
 class QProgressBar;
 class QStackedWidget;
 class QToolButton;
@@ -61,6 +62,9 @@ private:
     QWidget* buildPanelsRibbonPage();
     void updateWindowTitle();
     QToolButton* makePanelToggle(const QString& text, QDockWidget* dock);
+    // Rebuilds `menu` from the persisted recent-projects list. Wired to the
+    // menu's aboutToShow so it always reflects the current MRU state.
+    void populateRecentMenu(QMenu* menu);
 
 private:
     QLabel* status_mode_label_ = nullptr;