fix: add #page-notification to container_editor.html for XBlock error display

[Anas12091101]

Description

The container_editor.html template was missing a #page-notification element.
This template is used when the Course Authoring MFE opens legacy XBlock editors
(like ORA) in a ModalIframe via /xblock/{id}/action/edit. Without this element,
the NotificationView had no DOM target, causing XBlock runtime error notifications
(e.g. "Validation error: dates out of order") to fail silently on save.

Useful information to include:

• Which edX user roles will this change impact? Course Author
• Include screenshots for changes to the UI (ideally, both "before" and "after" screenshots, if applicable).

• Provide links to the description of corresponding configuration changes. Remember to correctly annotate these
  changes.

Supporting information

https://github.com/mitodl/hq/issues/11318

Testing instructions

Testing Instructions:

1. Navigate to a course in Studio (via the Course Authoring MFE) and open a unit containing an ORA (Open Response Assessment) component, or create a new ORA problem

2. Click the Edit button on the ORA component to open the legacy editor modal.

3. In the ORA editor, configure step dates out of order — e.g., set a step's start date earlier than the previous step's start date.

4. Click Save.

5. Expected (after fix): A red error notification appears at the bottom of the editor modal with a message like "OpenAssessment Save Error: Validation error: This step's start date cannot be earlier than the previous step's start date." The XBlock data is not saved.

6. Expected (before fix): No error notification is shown. The save fails silently with no feedback to the user.

7. Verify that saving with valid dates still works correctly — the modal closes and the component refreshes.

Deadline

"None"

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