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".

ios/Classes/ClipboardPlugin.swift

@@ -0,0 +1,52 @@
+import Flutter
+import MobileCoreServices
+import UIKit
+import UniformTypeIdentifiers
+
+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 = UIPasteboard.general
+
+        // Prefer raw PNG / JPEG bytes already on the clipboard so we don't
+        // round-trip through UIImage when we don't have to.
+        if #available(iOS 14.0, *) {
+            if let png = pb.data(forPasteboardType: UTType.png.identifier) {
+                return result(FlutterStandardTypedData(bytes: png))
+            }
+            if let jpg = pb.data(forPasteboardType: UTType.jpeg.identifier) {
+                return result(FlutterStandardTypedData(bytes: jpg))
+            }
+        } else {
+            if let png = pb.data(forPasteboardType: kUTTypePNG as String) {
+                return result(FlutterStandardTypedData(bytes: png))
+            }
+            if let jpg = pb.data(forPasteboardType: kUTTypeJPEG as String) {
+                return result(FlutterStandardTypedData(bytes: jpg))
+            }
+        }
+
+        // Fallback: re-encode whatever UIImage resolves to as PNG.
+        if let img = pb.image, let png = img.pngData() {
+            return result(FlutterStandardTypedData(bytes: png))
+        }
+
+        result(nil)
+    }
+}

ios/Classes/UxPlugin.swift

@@ -10,6 +10,7 @@ public class UxPlugin: NSObject, FlutterPlugin {
             SensorPlugin(),
             FilePlugin(),
             ScannerPlugin(),
+            ClipboardPlugin(),
         ]
         for plugin in plugins {
             plugin.register(with: registrar)