Search: Add title/image to the overlay

Allow adding a title and/or image into the overlay so that the end user can brand the search experience for their end users.

Some ideas:
- Just take the header image from the main site and resize it
- Just take the title of the main site and add it into the overlay
- By default add the image and the title but have options to disable them or override them

The issue does not name any files, tests, or entry points. Start by locating the search overlay implementation and reviewing how the main site's title and header image are exposed; clarify the default behavior and opt-out or override options before defining completion criteria.

Written by the indexing model from the issue text.