clipboard: add UxClipboard.readImage native bridge

Flutter's Clipboard API only exposes text shapes. Banlu's chat composer
needs image bytes from the system clipboard for desktop paste, so add
a UxClipboard facade backed by per-platform native plugins:

* iOS: prefer raw PNG/JPEG bytes off the pasteboard, fall back to
  re-encoding `UIPasteboard.image` as PNG.
* macOS: prefer NSPasteboard `.png`, fall back to TIFF transcoded
  through NSBitmapImageRep so screenshots / Preview hand-offs still
  work.
* Android: read primary ClipData's first item URI and stream the bytes
  through ContentResolver — don't trust the clip description's MIME,
  copy whatever the resolver returns.

Returns null (never throws) when the clipboard has no image — callers
treat null as "fall through to text paste".

macos/Classes/UxPlugin.swift

@@ -7,6 +7,7 @@ public class UxPlugin: NSObject, FlutterPlugin {
     public static func register(with registrar: FlutterPluginRegistrar) {
         plugins = [
             FilePlugin(),
+            ClipboardPlugin(),
         ]
         for plugin in plugins {
             plugin.register(with: registrar)