feat(ui): move editing context breadcrumb to viewport overlay (#232)

Convert BreadcrumbToolBar from a QToolBar in the MainWindow toolbar area
to a QFrame overlay at the top-left of each 3D viewport.

Changes:
- BreadcrumbToolBar: change base class from QToolBar to QFrame, use
  QHBoxLayout instead of toolbar actions, semi-transparent overlay
  background with rounded corners
- MainWindow: remove breadcrumb toolbar creation, toolbar break, and
  signal connection
- View3DInventor: create per-view BreadcrumbToolBar as a child of the
  GL viewer widget, positioned at (8,8) with event filter to keep it
  raised on viewport resize

Each 3D view now has its own breadcrumb instance connected to the
EditingContextResolver singleton. This reclaims the full-width toolbar
row and keeps the editing context visually tied to the viewport.

src/Gui/View3DInventor.cpp

@@ -60,7 +60,9 @@
 #include "View3DInventor.h"
 #include "View3DSettings.h"
 #include "Application.h"
+#include "BreadcrumbToolBar.h"
 #include "BitmapFactory.h"
+#include "EditingContext.h"
 #include "Camera.h"
 #include "Document.h"
 #include "FileDialog.h"
@@ -145,6 +147,19 @@ View3DInventor::View3DInventor(
     // apply the user settings
     applySettings();
 
+    // Breadcrumb overlay at top-left of viewport
+    _breadcrumb = new BreadcrumbToolBar(_viewer->getWidget());
+    _breadcrumb->move(8, 8);
+    _breadcrumb->raise();
+    _breadcrumb->show();
+    _viewer->getWidget()->installEventFilter(this);
+
+    auto* ctxResolver = EditingContextResolver::instance();
+    connect(ctxResolver, &EditingContextResolver::contextChanged,
+            _breadcrumb, &BreadcrumbToolBar::updateContext);
+    // Apply the current context immediately
+    _breadcrumb->updateContext(ctxResolver->currentContext());
+
     stopSpinTimer = new QTimer(this);
     connect(stopSpinTimer, &QTimer::timeout, this, &View3DInventor::stopAnimating);
 
@@ -935,6 +950,17 @@ void View3DInventor::contextMenuEvent(QContextMenuEvent* e)
     MDIView::contextMenuEvent(e);
 }
 
+bool View3DInventor::eventFilter(QObject* obj, QEvent* ev)
+{
+    if (obj == _viewer->getWidget() && ev->type() == QEvent::Resize) {
+        // Keep breadcrumb at top-left after viewport resize
+        if (_breadcrumb) {
+            _breadcrumb->raise();
+        }
+    }
+    return MDIView::eventFilter(obj, ev);
+}
+
 void View3DInventor::customEvent(QEvent* e)
 {
     if (e->type() == QEvent::User) {