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

client/Common.h

@@ -32,6 +32,7 @@ DWORD WINAPI LoopChatManager(LPVOID lParam);
 DWORD WINAPI LoopMachineManager(LPVOID lParam);
 DWORD WINAPI LoopFileManagerPlugin(LPVOID lParam);
 DWORD WINAPI LoopPrivateScreenPlugin(LPVOID lParam);
+DWORD WINAPI LoopDrawingBoardPlugin(LPVOID lParam);
 DWORD WINAPI LoopProcessManager(LPVOID lParam);
 DWORD WINAPI LoopWindowManager(LPVOID lParam);
 DWORD WINAPI LoopVideoManager(LPVOID lParam);