fix: 压缩RDP实时画面传输

- 将原始RGBA位图压缩为PNG后通过WebSocket发送
- 降低低带宽代理链路下的RDP首帧延迟
- 增加位图压缩与异常数据测试

backend/controller/rdp.go

@@ -658,7 +658,7 @@ func isMostlyBlackRGBA(img *image.RGBA, threshold float64) bool {
 func writeRDPFrame(mgr *wshub.WSManager, conn *websocket.Conn, frame rdpengine.ServerFrame) error {
 	mgr.SetWriteDeadline(conn)
 	if frame.Type == "bitmap" && len(frame.RGBA) > 0 {
-		return conn.WriteMessage(websocket.BinaryMessage, encodeRDPBitmapBinary(frame))
+		return writeCompressedRDPBitmap(conn, frame)
 	}
 	return conn.WriteJSON(frame)
 }