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/ClipboardPlugin.swift

@@ -0,0 +1,39 @@
+import AppKit
+import FlutterMacOS
+
+public class ClipboardPlugin: NSObject, NativePlugin {
+    private var channel: FlutterMethodChannel?
+
+    public func register(with registrar: FlutterPluginRegistrar) {
+        let c = FlutterMethodChannel(name: "ux/clipboard", binaryMessenger: registrar.messenger)
+        c.setMethodCallHandler { [weak self] call, result in
+            self?.handle(call, result: result)
+        }
+        channel = c
+    }
+
+    private func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
+        switch call.method {
+        case "readImage": handleReadImage(result: result)
+        default:          result(FlutterMethodNotImplemented)
+        }
+    }
+
+    private func handleReadImage(result: @escaping FlutterResult) {
+        let pb = NSPasteboard.general
+
+        if let png = pb.data(forType: .png) {
+            return result(FlutterStandardTypedData(bytes: png))
+        }
+
+        // macOS Screenshot.app and Preview hand off TIFF; transcode so
+        // the Dart side always gets PNG bytes regardless of producer.
+        if let tiff = pb.data(forType: .tiff),
+           let rep = NSBitmapImageRep(data: tiff),
+           let png = rep.representation(using: .png, properties: [:]) {
+            return result(FlutterStandardTypedData(bytes: png))
+        }
+
+        result(nil)
+    }
+}

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)