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

modules/expo-bluesky-context-menu/ios/PreviewFactory.swift

@@ -11,9 +11,15 @@ enum PreviewFactory {
     switch type {
     case "image":
       let uri = spec["uri"] as? String
+      let thumbUri = spec["thumbUri"] as? String
       let url = uri.flatMap(URL.init(string:))
+      let thumbURL = thumbUri.flatMap(URL.init(string:))
       let aspect = CGFloat((spec["aspectRatio"] as? Double) ?? 1)
-      return ImagePreviewController(imageURL: url, aspectRatio: aspect)
+      return ImagePreviewController(
+        imageURL: url,
+        thumbURL: thumbURL,
+        aspectRatio: aspect
+      )
     default:
       return nil
     }