feat: decouple notifications panel using widget registry mechanism (#1885)

Removes the tightly coupled upgrade/upsell ("notifications") panel from the Learning MFE core and replaces it with a pluggable widget registry system. The right sidebar now supports dynamically-registered external widgets, making it easy to add, remove, or replace sidebar panels without forking the MFE.

example.env.config.jsx

@@ -1,9 +1,12 @@
 import UnitTranslationPlugin from '@edx/unit-translation-selector-plugin';
+import { upgradeWidgetConfig } from './src/widgets/upgrade/src/index';
 import { PLUGIN_OPERATIONS, DIRECT_PLUGIN } from '@openedx/frontend-plugin-framework';
 
 // Load environment variables from .env file
 const config = {
   ...process.env,
+  SIDEBAR_WIDGETS: [upgradeWidgetConfig],
+
   pluginSlots: {
     unit_title_plugin: {
       plugins: [