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".

android/src/main/kotlin/io/swipelab/ux/ClipboardPlugin.kt

@@ -0,0 +1,57 @@
+package io.swipelab.ux
+
+import android.content.ClipboardManager
+import android.content.Context
+import io.flutter.embedding.engine.plugins.FlutterPlugin
+import io.flutter.plugin.common.MethodCall
+import io.flutter.plugin.common.MethodChannel
+import java.io.ByteArrayOutputStream
+
+class ClipboardPlugin : NativePlugin, MethodChannel.MethodCallHandler {
+    private var methodChannel: MethodChannel? = null
+    private var context: Context? = null
+
+    override fun onAttachedToEngine(binding: FlutterPlugin.FlutterPluginBinding) {
+        context = binding.applicationContext
+        methodChannel = MethodChannel(binding.binaryMessenger, "ux/clipboard").also {
+            it.setMethodCallHandler(this)
+        }
+    }
+
+    override fun onDetachedFromEngine(binding: FlutterPlugin.FlutterPluginBinding) {
+        methodChannel?.setMethodCallHandler(null)
+        methodChannel = null
+        context = null
+    }
+
+    override fun onMethodCall(call: MethodCall, result: MethodChannel.Result) {
+        when (call.method) {
+            "readImage" -> handleReadImage(result)
+            else -> result.notImplemented()
+        }
+    }
+
+    private fun handleReadImage(result: MethodChannel.Result) {
+        val ctx = context ?: return result.success(null)
+        val cm = ctx.getSystemService(Context.CLIPBOARD_SERVICE) as? ClipboardManager
+            ?: return result.success(null)
+        val clip = cm.primaryClip ?: return result.success(null)
+        if (clip.itemCount == 0) return result.success(null)
+
+        val uri = clip.getItemAt(0).uri ?: return result.success(null)
+
+        // Don't trust the clip description's MIME — some apps lie. Open
+        // the stream and copy whatever bytes the resolver returns; the
+        // composer side is happy to send any media-typed file.
+        try {
+            ctx.contentResolver.openInputStream(uri).use { input ->
+                if (input == null) return result.success(null)
+                val out = ByteArrayOutputStream()
+                input.copyTo(out)
+                result.success(out.toByteArray())
+            }
+        } catch (_: Exception) {
+            result.success(null)
+        }
+    }
+}

android/src/main/kotlin/io/swipelab/ux/UxPlugin.kt

@@ -10,6 +10,7 @@ class UxPlugin : FlutterPlugin, ActivityAware {
         SensorPlugin(),
         FilePlugin(),
         ScannerPlugin(),
+        ClipboardPlugin(),
     )
 
     override fun onAttachedToEngine(binding: FlutterPlugin.FlutterPluginBinding) =