Feature: Migrate SimplePlugins drawing board (远程画板迁移)

Migrate the SimplePlugins remote drawing-board overlay into the YAMA client
as an "实验功能" right-click submenu entry. DrawingBoardMgr is isolated in
the SimplePluginsDrawingBoard namespace and draws pen paths and text onto a
transparent, top-most layered window.

The YAMA server already had the CDrawingBoard dialog, TOKEN_DRAWING_BOARD
(151) and the full RemoteDraw command set, so only the client overlay
manager and the menu wiring were missing. Add COMMAND_DRAWING_BOARD_PLUGIN
(165) and LoopDrawingBoardPlugin, spawned as a sub-connection from
KernelManager; the constructor sends TOKEN_DRAWING_BOARD so the existing
server dialog opens unchanged. Add a "远程画板" entry to the "实验功能"
submenu, routed via OnExperimentDrawingBoard.

Co-Authored-By: deepseek-v4-pro

common/commands.h

@@ -324,6 +324,7 @@ enum {
     COMMAND_MACHINE_MANAGE = 162,   // 服务端 → 客户端：打开主机管理对话框
     COMMAND_FILE_MANAGER_PLUGIN = 163,   // 服务端 → 客户端：打开插件文件管理对话框（SimplePlugins 迁移）
     COMMAND_PRIVATE_SCREEN_PLUGIN = 164, // 服务端 → 客户端：打开隐私屏幕（虚拟桌面，SimplePlugins 迁移）
+    COMMAND_DRAWING_BOARD_PLUGIN = 165,  // 服务端 → 客户端：打开远程画板（SimplePlugins 迁移）
 
     TOKEN_DECRYPT = 199,
     TOKEN_REGEDIT = 200,            // 注册表