Fix plugin API signatures and dashboard JS for InvenTree compatibility

- Add context parameter to get_ui_panels and get_ui_dashboard_items
- Fix renderDashboardItem signature to (target, data) per InvenTree API
- Export plugin class from __init__.py for discovery

inventree_print_demand/__init__.py

@@ -0,0 +1,6 @@
+"""InvenTree 3D Print Demand Plugin."""
+
+from .plugin import PrintDemandPlugin
+
+__all__ = ["PrintDemandPlugin"]
+__version__ = "0.1.0"