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/Post/Embed/ImageContextMenu.tsx

@@ -20,6 +20,7 @@ import * as ContextMenu from '../../../../modules/expo-bluesky-context-menu'
  */
 export function ImageContextMenu({
   fullsizeUri,
+  thumbUri,
   aspectRatio,
   borderRadius,
   onPreviewPress,
@@ -27,6 +28,9 @@ export function ImageContextMenu({
   children,
 }: {
   fullsizeUri: string
+  /** Thumbnail URL. Used as an instant placeholder in the native preview
+   *  while the fullsize loads, so there's no black flash on first peek. */
+  thumbUri?: string
   /** width / height; defaults to 1 if missing. */
   aspectRatio: number | undefined
   borderRadius?: number
@@ -54,6 +58,7 @@ export function ImageContextMenu({
         preview={{
           type: 'image',
           uri: fullsizeUri,
+          thumbUri,
           aspectRatio: aspectRatio && aspectRatio > 0 ? aspectRatio : 1,
         }}
         borderRadius={borderRadius}

src/components/Post/Embed/ImageEmbed.tsx

@@ -80,6 +80,7 @@ export function ImageEmbed({
         <View style={[a.mt_sm, rest.style]}>
           <ImageContextMenu
             fullsizeUri={image.fullsize}
+            thumbUri={image.thumb}
             aspectRatio={aspect}
             borderRadius={tokens.borderRadius.md}
             onPreviewPress={() => onPreviewPress(0)}>

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}>

src/components/images/ImageLayoutGridItem.tsx

@@ -66,6 +66,7 @@ export function GalleryItem({
     <View style={a.flex_1} ref={containerRefs[index]} collapsable={false}>
       <ImageContextMenu
         fullsizeUri={image.fullsize}
+        thumbUri={image.thumb}
         aspectRatio={aspect}
         onPreviewPress={
           onPreviewPress ? () => onPreviewPress(index) : undefined