Wire image context menu into grid and gallery embeds

Thread onPreviewPress(index) through ImageLayoutGrid / GalleryItem and the
Gallery scroll item so multi-image embeds get the same iOS peek behaviour as
single-image embeds. Each grid cell wraps its Pressable in ImageContextMenu
with the image's true aspect ratio, so the preview lifts to the full
uncropped image even when the cell is cropped to square.

https://claude.ai/code/session_015REmux3R9uuEMMJUHxTyQT

src/components/images/ImageLayoutGrid.tsx

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