fix(instantpage): V2 slideshow background is black, not the media placeholder color

The slideshow container + each page's letterbox used theme.panelSecondaryColor
(reading as the media-placeholder gray); a slideshow should render on a black
gallery backdrop. Sets the container background (init + update) and each
pageView background to black.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>