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.
