fix(instantpage): V2 slideshow black image backdrop + current-page-only gallery transition

Two InstantView-V2 slideshow rendering corrections:

- Black backdrop: the visible fill behind a slideshow photo is the image node's
  emptyColor, which for the rich-message theme is panelBackgroundColor
  (accentControlColor·0.1) — the placeholder-gray. Add a scoped emptyColor
  override to InstantPageImageNode (threaded via makeMediaWrapper, default nil =
  unchanged for all other callers) and pass .black from the slideshow, so the
  backdrop is a proper gallery black rather than the placeholder color. Pairs
  with the earlier SlideshowView/pageView .black backgrounds.

- Gallery close: a slideshow registers under EVERY contained media index, so the
  gallery could ask it to animate in/out to a media sitting on an off-screen
  page. Track the current page and return a transition node only for the media on
  the currently-displayed page (nil otherwise → the gallery uses a plain fade
  instead of flying to the wrong place).

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