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.

docs/decisions/0010-upgrade-widget-extraction.md

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+# 10. Extract Upgrade Widget from Core MFE
+
+## Context
+
+The Learning MFE shipped with a built-in upgrade sidebar widget internally named "Notifications". This caused several problems:
+
+1. **Confusing naming**: The "Notifications" label implied platform notification functionality, conflicting with the actual notification tray used elsewhere in the platform.
+2. **Core/commercial coupling**: The upgrade widget is a commercial feature specific to paid instances, not part of the Open edX core offering. Yet it was enabled by default for all deployments.
+3. **Empty panel problem**: In courses without a paid track, the upgrade panel was empty but still visible, degrading the learner experience.
+4. **Tight coupling**: The widget was directly imported and wired into the sidebar, making it difficult to customize or disable without forking the MFE.
+
+## Decision
+
+### 1. Rename "notifications" terminology to "upgrade"
+
+All internal code references to "notification" that specifically relate to the upgrade panel are renamed to "upgrade":
+
+- `WIDGETS.NOTIFICATIONS` → `WIDGETS.UPGRADE`
+- `notificationStatus` context property → `upgradeWidgetStatus`
+- `onNotificationSeen` → `onUpgradeWidgetSeen`
+- `NotificationTray` component → `UpgradePanel`
+- `NotificationTrigger` component → `UpgradeTrigger`
+- `NotificationIcon` component → `UpgradeIcon`
+- localStorage keys: `notificationStatus.${courseId}` → `upgradeWidget.${courseId}`
+- i18n message keys (`notification.*`)
+
+
+### 2. Widget moved to `src/widgets/upgrade/`
+
+Rather than publishing a separate npm package, the widget is kept as a subdirectory of the MFE. This avoids monorepo publishing overhead while maintaining a clean separation from the sidebar framework.
+
+Instances that need the upgrade panel register it via `env.config.jsx`:
+
+```js
+import upgradeWidget from './src/widgets/upgrade/src/index';
+
+export const SIDEBAR_WIDGETS = [upgradeWidget];
+```