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/static/print_demand/dashboard.js

@@ -5,8 +5,11 @@
  * a colour-coded table showing stock vs demand for every 3D-printed part.
  */
 
-export function renderDashboardItem(context) {
-    const target = context.target;
+export function renderDashboardItem(target, data) {
+    if (!target) {
+        console.error("No target provided to renderDashboardItem");
+        return;
+    }
 
     target.innerHTML = '<em>Loading 3D print demand data...</em>';