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/ExpoBlueskyContextMenu.podspec

@@ -10,6 +10,8 @@ Pod::Spec.new do |s|
   s.static_framework = true
 
   s.dependency 'ExpoModulesCore'
+  # Must match the version pinned by expo-image so we share SDImageCache.shared.
+  s.dependency 'SDWebImage', '~> 5.21.0'
 
   s.pod_target_xcconfig = {
     'DEFINES_MODULE' => 'YES',