Pass thumbnail ref through peek commit so lightbox return animates

The lightbox's close animation needs the original thumbnail ref/dims to know
where to animate back to. The peek-commit path was passing null, so the
lightbox dropped to a fade instead of the usual return-to-thumb animation.

Rework each embed path so the tap handler and the peek-commit handler
invoke the same openLightbox call with the same ref + dims:

- Grid and scrollable Gallery: derive `openLightboxAtIndex` once per cell
  and hand it to both Pressable.onPress and ContextMenu.onPreviewPress.
- Single image: add onContainerRef / onDimsChange callbacks to
  AutoSizedImage, capture them in ImageEmbed, and use them for the peek
  commit. Drops the now-redundant onPreviewPress prop chain.

https://claude.ai/code/session_015REmux3R9uuEMMJUHxTyQT

src/components/images/ImageLayoutGrid.tsx

@@ -17,7 +17,6 @@ interface ImageLayoutGridProps {
   ) => void
   onLongPress?: (index: number) => void
   onPressIn?: (index: number) => void
-  onPreviewPress?: (index: number) => void
   style?: StyleProp<ViewStyle>
   viewContext?: PostEmbedViewContext
 }
@@ -49,7 +48,6 @@ interface ImageLayoutGridInnerProps {
   ) => void
   onLongPress?: (index: number) => void
   onPressIn?: (index: number) => void
-  onPreviewPress?: (index: number) => void
   viewContext?: PostEmbedViewContext
   gap: {gap: number}
 }