Show "Review page" in tree view to help set expectations + make it optional

## Description
When a form has multiple pages, a review page is added to the form at the final step to allow users to check their answers before submitting. Currently this is visible in "Branching" or logic mapping view where the form flow is visualized, but not in "Questions" or tree view where the page outline exists. This creates surprise and confusion for users.

Related #3960

## Steps to reproduce
1. Go to GC Forms --> Staging
2. Create a form
3. Add a new page
4. See all pages on the form set-up right panel
5. See how review page is missing
6. Navigate to "Branching"
7. See how review page is inserted

## Details
- **Browser**: Chrome
- **Operating system**: Mac
- **Environment**: Staging
- **Language**: English and French
- **Viewport**: Desktop

## Expected behaviour
- Matching form outline (tree view) and form mapping (logic view).
- Review page shows up as locked element that cannot be edited (much like the confirmation page heading).
- OR Review page is an optional page that clients can choose to add to their form if it makes sense (opt in/out)

## Screenshots or videos