Add iOS peek long-press context menu for image embeds

Introduces expo-bluesky-context-menu, a compositional Root/Trigger/Menu/MenuItem
wrapper around UIContextMenuInteraction. The preview ViewController is sized to
the image's true aspect ratio so tall/panorama previews don't stretch mid-lift,
and menu icons are rasterized from the app's SVG icon set on the native side.

The preview prop is a discriminated union (image today; video and externalCard
reserved) so the same module can back those embeds in follow-ups.

https://claude.ai/code/session_015REmux3R9uuEMMJUHxTyQT

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

@@ -0,0 +1,21 @@
+import UIKit
+
+/// Decodes the `preview` prop shipped from JS and constructs the right
+/// `UIViewController` for the peek. Day-one only handles `image`. Add cases
+/// here for `video` and `externalCard` follow-ups.
+enum PreviewFactory {
+  static func makeController(from spec: [String: Any]?) -> UIViewController? {
+    guard let spec = spec,
+          let type = spec["type"] as? String else { return nil }
+
+    switch type {
+    case "image":
+      let uri = spec["uri"] as? String
+      let url = uri.flatMap(URL.init(string:))
+      let aspect = CGFloat((spec["aspectRatio"] as? Double) ?? 1)
+      return ImagePreviewController(imageURL: url, aspectRatio: aspect)
+    default:
+      return nil
+    }
+  }
+}