Share expo-image cache; thumb placeholder; smoother dismiss

Two UX fixes for the peek preview:

1. Black flash on first peek — the preview VC now depends on SDWebImage
   directly (same version pinned by expo-image) and reads from
   `SDImageCache.shared`. On open it paints the fullsize synchronously if it's
   cached (common, since onPressIn prefetches), otherwise paints the thumb as a
   placeholder while the fullsize loads and swaps in. Also switches the image
   view to autoresizingMask so the dismiss animation interpolates cleanly
   without AutoLayout relayout mid-animation.

2. Two-stage dismiss animation on aspect-mismatched thumbnails — drop the
   explicit `previewForDismissingMenuWithConfiguration` override. With only
   the highlight provider implemented iOS reverses it end-to-end, which
   interpolates size and position together instead of snapping back to the
   preview's original size before translating.

https://claude.ai/code/session_015REmux3R9uuEMMJUHxTyQT

src/components/images/Gallery/index.tsx

@@ -426,6 +426,7 @@ function GalleryImage({
       aria-label={image.alt || l`Image ${index + 1} of ${imageCount}`}>
       <ImageContextMenu
         fullsizeUri={image.fullsize}
+        thumbUri={image.thumb}
         aspectRatio={aspectRatio}
         borderRadius={tokens.borderRadius.md}
         onPreviewPress={onPreviewPress}>