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/plugin.py

@@ -38,10 +38,10 @@ class PrintDemandPlugin(SettingsMixin, UrlsMixin, UserInterfaceMixin, InvenTreeP
             re_path(r'^api/demand/', self.api_demand, name='api-demand'),
         ]
 
-    def get_ui_panels(self, request, **kwargs):
+    def get_ui_panels(self, request, context, **kwargs):
         return []
 
-    def get_ui_dashboard_items(self, request, **kwargs):
+    def get_ui_dashboard_items(self, request, context, **kwargs):
         return [
             {
                 'key': 'print-demand',