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".

lib/src/clipboard.dart

@@ -0,0 +1,28 @@
+import 'package:flutter/services.dart';
+
+/// OS clipboard access for shapes Flutter's [Clipboard] doesn't cover.
+/// Right now this is image bytes — the system text path is already
+/// handled by the SDK's `Clipboard.getData(Clipboard.kTextPlain)`.
+class UxClipboard {
+  UxClipboard._();
+
+  static const _channel = MethodChannel('ux/clipboard');
+
+  /// Returns the image currently on the system clipboard as raw bytes
+  /// (PNG on iOS / macOS / Android), or `null` if no image is available.
+  ///
+  /// On macOS the call also handles TIFF clipboards by transcoding to
+  /// PNG; Android resolves `content://` URIs through ContentResolver and
+  /// returns the underlying image bytes verbatim. Never throws — channel
+  /// failures resolve to `null`.
+  static Future<Uint8List?> readImage() async {
+    try {
+      final bytes = await _channel.invokeMethod<Uint8List>('readImage');
+      return bytes;
+    } on PlatformException {
+      return null;
+    } on MissingPluginException {
+      return null;
+    }
+  }
+}