[Site Design Revamp] Create carousel view for previewing design categories

Description

A carousel / paging view will allow the user to horizontally swipe between full-screen / modal previews of designs in a category.

Current flow:

1. User taps on a design on the Design Screen main view
2. Action buttons appear from the bottom ("Preview", "Choose")
3. User taps "Preview" button
4. The chosen design's preview is shown in a modal / full screen
5. The user can swipe down to dismiss the preview

New flow:

1. User taps on a design on the Design Screen main view
2. A horizontal carousel / paging view is shown
3. The carousel view contains all the designs in the tapped design's category
4. The carousel view is focused / centered on the tapped design
5. The user can swipe horizontally to access preceding and succeeding designs from the category
6. The user can swipe down to dismiss the carousel view

Technical considerations:

• The carousel view may take an array of preview views, and a pointer to the starting view
• The carousel view may be responsible for firing the "Preview Viewed" event
• This view will support scrolling in a web view, horizontal scrolling between previews, and a downward gesture to dismiss itself

PRs should target the feature/site-design-revamp feature branch.

Start by locating the Design Screen and its existing modal preview flow described in the issue, then inspect the feature/site-design-revamp branch. Done means a centered, horizontally swipeable carousel shows all designs in the selected category, supports scrolling previews and downward dismissal, and preserves the stated starting position.